Accurizing is the process of improving the accuracy of a
firearmor airgun.cite web |url=http://saami.org/Glossary/index.cfm |title=SAAMI glossary |accessdate=2007-08-30]
For firearms, accuracy is defined as the ability to hit exactly what you're aiming at. Further,
accuracy and precision, or the ability to hit the same place over and over again in a repeatable fashion, are the goals of accurizing. [" [http://www.chuckhawks.com/accuracy_gun_cartridge_shooter.htm Accuracy: Gun, Cartridge, and Shooter] " by Bob Beers at Chuck Hawks web site]
Accurizing generally concentrates on three different areas:
*Usability: Enhancements that give the shooter a more consistent hold on the firearm, and a more consistent trigger pull.
*Tolerances: Parts that fit together better will shift less or shift more consistently under recoil. Moving parts still have to move, however, and finding the right compromise is more art than science.
*Harmonics: Barrels vibrate under the shock of firing. These vibrations can be minimized or tuned to minimize their impact on accuracy. Generally the harmonic effects are proportional to the square of the barrel length, and so are generally only of concern in
rifles but not handguns.And, applicable only to airguns:
*Consistent power: since airguns provide the propulsive force themselves, rather than relying on the ammunition as firearms do, the source of this power is open to accurizing operations.
The key to an accurate firearm is consistency. Getting everything to happen the same way for every shot is key to producing small groups, and there are a large number of issues to be addressed in achieving an accurate firearm.cite web |url=http://www.tacticaloperations.com/swatapril2000/index.html |title=Tack Driving Tactical Rifle from Tac Ops|accessdate=2007-08-30] cite web |url=http://www.bullseyepistol.com/cstomgun.htm |title=The Custom-Built Handgun|accessdate=2007-08-30] cite web |url=http://outlands.tripod.com/armory/mini-14.htm |title=Accurizing the
Mini-14|accessdate=2007-08-30] cite web |url=http://www.gun-tests.com/performance/oct97shootbetter.html |title=Gun-Tests.com Accurizing the Beretta 92FS|accessdate=2007-08-30] The keys to firing an accurate shot are a firm but not overtight grip, the ability to get a good sight picture, and a controlled squeeze of the trigger. The ability to manage recoil is also important in heavily recoiling calibers, both to aid in possible additional shots, and to prevent the user from developing a fear of the recoil.cite web |url=http://www.bullseyepistol.com/goodgrip.htm |title=The Benefits of Custom Grips by John Dreyer, Encyclopedia of Bullseye Pistol|accessdate=2007-08-30]
Determining accuracy is not always a straightforward task, as it depends on a large number of variables.cite web |url=http://www.chuckhawks.com/accuracy_gun_cartridge_shooter.htm |title=Accuracy: Gun, Cartridge, and Shooter |author=Bob Beers |publisher=Chuck Hawks |accessdate=2007-09-04]
Since adjusting the point of impact to match the point of aim is relatively simple with any type of adjustable sights, the primary goal of accurizing is to increase the precision of the firearm, which is generally measured by looking at the dispersion of a number of shots fired at the same point of aim. An ideal group would be one where all shots land in a hole no larger than the diameter of a single bullet; this would indicate zero dispersion. The most common way of measuring groups then is to measure the edge to edge distance of the farthest holes, and subtract the bullet diameter, which gives the "center to center" or "c-t-c" measurement of the group. This can be expressed in linear measures ("a one inch group at 100 yards") or in angular measures ("a MOA group"). Groups for rifles are traditionally shot at 100 yards, where a minute of arc equals just over one inch, and the one MOA group is a traditional
benchmarkof accuracy. Handguns are generally used at closer ranges, and are tested for accuracy at their intended range of use. Also of importance is the number of shots fired. Statistical likelihood says the fewer shots that are fired, the smaller the dispersion will be. Three shots is generally the minimum fired, though a five or ten shot group will provide a more realistic measurement of the firearm's precision.cite web |url=http://www.snipercountry.com/compendium/Comp_G.htm |title="G" |accessdate=2007-09-06]
Even defining accuracy can be problematic. An example of this can be shown by the following tests, run by "Performance Shooter" magazine in December, 1996. The magazine was testing seven brands of
.38 Special wadcutterrounds in three different revolvers, a Smith & WessonModel 686 and Model 52, and a Colt PythonTarget model, with six, five and eight inch long barrels, respectively. Ten groups of five shots were fired and measured from each revolver with each ammunition. Click on the image at right to see a larger view of the graph of average group sizes for each type of ammunition and each revolver. The average group size for the overall test was 2.85 inches (72.4 mm).cite web |url=http://www.gun-tests.com/performance/dec96specialammo.html |title=Performance Shooter .38 Special wadcutter tests |accessdate=2007-08-30]
averagegroup size, the winner was the Model 686, which shot an average group of 2.69 inches across the brands of ammunition, with a standard deviation between ammunition types of 0.54 inches. However, the Model 52, while shooting slightly larger groups at 2.88 inches, was far more consistent across the brands, with a standard deviationof only 0.30 inches, and was the most consistent performer of the test. But, if you were able to tune your ammunition to the gun, the clear winner was the Python, which averaged just 1.69 inches with its favored brand of ammunition. The Python was also by far the pickiest, however, turning in the largest groups at 6.08 inch and 4.0 inch averages with its least favorite brands, for a standard deviation of 1.6 inches.
Based on this test, answering the question "Which is the most accurate?" becomes a matter of opinion. The answer that would most likely be given would probably be the 686, as it shot the best average groups. However, as the Python showed the best performance with one brand of ammunition, it might be the best choice if that brand of ammunition were acceptable for the application in question. If a consistent supply of ammunition were a problem, then the 52 might be the best choice, since it showed the least sensitivity to differences in ammunition.
As the goal of accurizing a firearm is to improve its accuracy, the way accuracy is measured becomes important. A firearm used primarily as a
hunting weaponwill need to be accurate on the first shot from a cold, clean barrel, while one used for target shooting may be allowed fouling shots before the first shot for record is fired. Issues of portability or restrictions of certain competitions may limit the alterations that can be made. In addition, every firearm is different, and processes that yield huge results on one may have no impact on anothercite web |url=http://www.chuckhawks.com/affordable_accuracy.htm |title=Affordable Accuracy |author=Bill Johnson |publisher=Chuck Hawks |accessdate=2007-09-04]
Another issue in measuring accuracy is the position in which the shots are fired. The most accurate shooting position is a supported position, such as firing from a benchrest with the firearm well supported by a shooting rest or sandbags; this eliminates much of the shooter's potential for error and will generally result in much smaller groups than firing from an unsupported position. Even for a firearm that is going to be shot offhand, accuracy testing from a machine rest will provide an idea of the ultimate attainable accuracy.cite web |url=http://www.chuckhawks.com/shooting_unsupported_positions.htm |title=Practical Accuracy in the Field: Shooting from Unsupported Positions |author=Chuck Hawks |accessdate=2007-09-06]
Factors affecting accuracy
The accuracy of a shot relies on many different factors, which can be broken down into three broad categories: the firearm, the cartridge, and the shooter. Accurizing generally refers to the processes that are applied to the firearm. Techniques relating to producing accurate ammunition are covered in internal and
external ballistics, and handloading, and just like accurizing a firearm, the goal is to produce the most consistent possible results. The shooter must also be consistent, and this means that the fundamentals of marksmanshiphave to be followed rigorously; any failure on the part of the shooter to remain focused and consistent can result in a bad shot.
No matter what the potential accuracy of a firearm, it cannot be fired accurately unless the shooter can fire it accurately. A firearm that is comfortable, fits the user well, and provides for a careful, consistent trigger pull and recoil management is not only a physical advantage over a poorly fit firearm, but a psychological one as well. Aesthetics may also contribute to usability, as an attractive firearm can help build confidence.
The dynamics of the trigger, known as its "pull", are one of the most important aspects of usability, since any movement of the firearm caused by pulling the trigger can have an impact on the placement of the shot. Trigger pulls are relative, however. Compare a sport like
action shooting, which emphasizes speed and uses relatively close targets with large scoring zones on the targets, to bullseyeshooting, which uses distant targets with tiny scoring zones. While both types of trigger need a predictable pull, bullseye shooters demand a much higher degree of precision.
Analysis of the trigger pull
The trigger pull consists of three stages:
#takeup or pretravel, which is the movement of the trigger which happens before the sear moves
#break, the movement during which the trigger moves the sear to the point of release
#overtravel, which is the distance a trigger moves after the sear releases
The takeup is the least critical stage of the trigger pull, and individual preferences vary widely. Two stage triggers, for example, consist of a noticeable takeup, followed by a distinct increase in the force required to pull the trigger, followed by the break. A single stage trigger, on the other hand, has no discernible movement before the break. Fully adjustable triggers will provide a two stage pull, and the option of reducing the first stage travel to zero, essentially making the trigger a single stage trigger.
The break is a far more critical stage of the pull, as it happens just prior to the shot being fired. Here again, individual preferences vary; some shooters prefer a "soft" break, where there is a smooth but discernible amount of trigger travel during firing, while others prefer a "crisp" break, with a heavier weight and little or no discernible movement.
The overtravel can be the most critical factor in the trigger pull, as any movement caused at this point will happen as the shot is fired. This is especially important with firearms where there is a sudden release of resistance when the sear breaks, such as in
double actiontriggers. An overtravel stop will arrest the motion of the trigger just after the break, and prevent movement.cite web |url=http://findarticles.com/p/articles/mi_m0BTT/is_179_30/ai_n15862407 |title=Over-travel = bad |publisher=American Handgunner |month=Jan-Feb |year=2006 |accessdate=2007-08-30]
Improving the trigger pull
An adjustable trigger may have ways to adjust all of these stages, plus trigger location. For example, a first stage or takeup adjustment might include weight and travel, a second stage or sear engagement adjustment might include weight and travel, and a trigger stop adjustment would limit the overtravel.cite web |url=http://www.davidtubb.com/tcom_images/t2k_images/t2k_manual_8.html |title=Anschutz Trigger |accessdate=2007-08-30]
While adjustable triggers may provide the greatest level of control, much can be done with standard non-adjustable triggers. Careful hand fitting and polishing of parts, addition of high precision or adjustable aftermarket parts, or fabrication of new parts can greatly improve most triggers. Care should be taken, however, since trigger work requires a great deal of care and precision, and a bad trigger job can easily render a firearm highly unsafe or unusable.
Most manufacturers ship firearms with fairly heavy, nonadjustable triggers, colloquially known as "lawyer triggers".cite web |url=http://www.chuckhawks.com/stars_stripes_pistol.htm |title=Stars and Stripes Custom Pistol Ammunition |author=Chuck Hawks |accessdate=2007-08-30] This is out of concern for liability; firearms are inherently dangerous, and allowing the user to adjust the trigger, or even implying such adjustments can be made, exposes the manufacturer to lawsuit. Likewise, manufacturers of aftermarket parts expose themselves to similar liability issues.cite book |title=The Preparation of a Product Liability Case |author=Scott Baldwin, Francis E. McGovern, Francis Hare |year=1998 |publisher=Aspen Publishers |isbn=0735501459 |url=http://books.google.com/books?id=fPF18ZQmcJMC&pg=RA4-PT544&dq=overtravel+trigger&ei=8DPXRpDHDYf26wLt1sH2BA&sig=jxXRptpWvy6XfzTT1w9UC_weEho]
The firearm's sights allow the user to align the firearm with the target, and in some cases all that separates a "target" firearm from a standard model is improved sights.cite book |title=Home Gunsmithing Digest, 2nd Ed. |last=Steindler |first=Robert A. |year=MCMLXXVIII |publisher=DBI Books, Inc. |location=Northfield, Illinois Chapter 7, You Can Build an Overnight Target Pistol]
Adjustable sights are essential for most target shooting, as they allow the point of impact to be adjusted to fall in the middle of the target when variables such as target range and ammunition type vary. Firearms with sights that are non-adjustable, or only coarsely adjustable, will not be able to reliably center the point of impact on the target. In addition, good visibility and a crisp sight picture help the user use the sights more reliably.
Typical rifle hunting sights, consisting of a square blade or bead front sight and an open notch mounted in front of the action, are good for quick alignment, but are far from ideal for precision shooting. An aperture sight mounted on the receiver or tang of the rifle, closer to the shooter's eye, will provide a much longer sight radius, which improves the shooter's ability to precisely align the sights. These sights often provide far more precise and repeatable adjustments than stock open sights. Telescopic and red dot sights offer advantages to shooters with poorer eyesight, as they bring the target and reticle into the same plane of focus, as well as providing the option of magnification. The downside to these is the additional weight and bulk they add.cite web |url=http://www.chuckhawks.com/choosing_sights.htm |title=Choosing the Right Sight |author=Chuck Hawks |accessdate=2007-09-06]
Telescopic sight, red dot sight]
tocks and grips
A good stock or grip is one that allows the shooter to have a relaxed but firm grip on the firearm. This can range from minor changes such as texturing grip surfaces or adding a wide, beavertail type grip safety to a 1911, all the way to a custom built, anatomically designed grip that "fits like a glove". Key features are:
*Comfort. This allows the shooter to relax and concentrate on shooting.
*Control. The shooter must be able to hold the firearm on target, and provide consistent movement under recoil.
*Positioning. The shooter must be positioned correctly and consistently, allowing easy use of the sights, and a clean trigger pull.
Gripping surfaces, particularly on handguns, are often designed to provide a large degree of friction to prevent the shooter's grip from shifting. This can be done by either using a material that provides a high friction, such as
rubber, or by adding texture to the grips. Traditionally wood grips and stocks are provided with checkering, a process where "V" shaped grooves are cut into the wood at an angle to each other, leaving a pattern of pyramidal projections in the wood. Another process uses a punch to leave a random pattern of dents in the surface, called stippling; this process is better suited to complex curves than checkering, and is often found on anatomical grips. Handguns with plastic frames will often have stippling or checkering molded into the frame.
Grips for handguns, especially ones used in single handed disciplines such as bullseye and Olympic handgun shooting, are critical to good accuracy. The grip provides very little control compared to a rifle stock, and so a good fit is required to give the shooter control over the gun and isolate the trigger finger movement. Fortunately, aftermarket parts makers provide a wide range of grips for handguns, allowing shooters to find grips that are suitable for their hands.
Competition handgun grips are anatomically shaped, and are often custom fitted to the shooter's hand. An ideal grip will fit the contours of the shooter's grip, so that the knuckles fall into position the same way every time. The ideal grip should allow the shooter to repeatedly grasp the gun, bring it up to shooting position with eyes closed, and be correctly aligned and on target when they eyes are opened; this means the grip provides a consistent placement, and minimal corrections when aiming. Contrary to commonly stated rules of thumb, the handgun should not line up with the forearm when gripped, but rather point slightly outwards, so that when held to shoot, the sights line up with the shooter's eye. In cross-dominant shooters, the angle will be more pronounced.cite web |url=http://www.nygord-precision.com/grips.htm |title=Modifying Your Pistol's Grip |author=Don Nygord |publisher=Nygord Precision |accessdate=2007-09-04] cite web |url=http://www.nygord-precision.com/grips_b.htm |title=Modifying Your Pistol's Grip part II|author=Don Nygord |publisher=Nygord Precision |accessdate=2007-09-04] Thumbrests, finger grooves (if well fitted), and palm rests all provide control over recoil. An adjustable palm rest is a desirable feature as well, as it allows the grip to be adjusted to fit the shooter's hand as it swells and contracts over time.cite web |url=http://bullseyepistol.com/goodgrip.htm |title=The Benefits of Custom Grips |author=John Dreyer |accessdate=2007-09-05]
Rifle and shotgun stocks
Stocks for long guns are not as often changed as handgun grips, but a well fitted stock can make a significant difference in accuracy. In shotguns in particular, the placement of the shooter's face on the stock provides the rear aiming point, and the correct drop and cast-off can greatly enhance accuracy. This was traditionally accomplished by steaming and bending the stock, but a simpler solution for modern guns is a set of shims that alter the angle of the stock.cite web |url=http://mossberg.com/scripts/prodList.asp?idCategory=16§ion=access |title=Stock Drop Spacers |publisher=O. F. Mossberg |accessdate=2007-09-05]
Rifle stocks have similar issues of fit, and though the use of sights makes this less vital than in a shotgun, a good fit still helps the shooter to relax and concentrate on fundamentals. The shape of a rifle stock should be suited to its intended use. High combs and vertical pistol grips are ideal for high mounted target sights or scopes and careful, deliberate shooting such as encountered in traditional target shooting,
metallic silhouette, or varmint hunting, as they provide a maximum point blank rangeand ideal trigger control. However, these features are not well suited for a typical hunting or action shootingrifle, where the rifle must be brought from a ready position to a shooting position quickly and smoothly. This use favors low sights or scopes, and a shallow pistol grip angle.cite book |title=Gunsmithing |author=Roy F. Dunlap |chapter=23 |publisher=Stackpole Books |isbn=0811707709 |year=1963] Rounded forearms are well suited for firing from offhand, while a square-bottomed forearm provides a stable base for shooting off of a sandbag or other rest.cite web |url=http://www.chuckhawks.com/rifle_stock.htm |title=Rifle Stocks|author=Chuck Hawks |publisher=Chuck Hawks |accessdate=2007-09-04]
Recoil is also a key issue in rifle stock design. Heavy recoiling rifles should have wide butts, with a good recoil pad to absorb the force of recoil, and a comb that is straight or slopes down towards the action, so that it does not push into the shooter's face under recoil. Other features of target or varmint stocks, such as thumbhole grips, can actually injure the shooter if used in a heavy recoiling hunting rifle.cite web |url=http://www.chuckhawks.com/rifle_recoil.htm |title=Rifle Recoil |author=Chuck Hawks |publisher=Chuck Hawks |accessdate=2007-09-04]
Some target shooting disciplines allow the use of various devices to help support the rifle, and these are often mounted to an accessory rail beneath the forend. "Target slings", unlike carrying slings, are used just on the off hand, usually with a "hand stop", and provide stability to the shooter's hold. "Palm rests" are another device that can be attached to the rail, to allow a shooter to lower their off hand and place their elbow into their body for support.cite web |url=http://www.championshooters.com/index-ssl.html |title=Champion Shooter's Supply |accessdate=2007-09-07] Target stocks also are available with a large degree of adjustment, including length of pull, drop, comb height and angle, and buttplate angle and curvature.cite web |url=http://www.championshooters.com/1907-2213big.htm |title=Anschutz 1907 with 2213 Stock |accessdate=2007-09-07]
The bullet does not leave the barrel as soon as the sear releases, rather there is a delay between the release of the sear and the bullet exiting the barrel. During this time any movement can move the firearm off target, and so this time should be minimized, especially for firearms that will be fired from an unsupported standing position. This delay can be broken down into two sections, the "lock time" and the "bullet dwell time".
Lock time is the time between the release of the sear, and the ignition of the cartridge. A lengthy lock time gives time for the shooter to drift off target, and so it is advantageous to minimize the lock time, and reduce the window for error. Reductions in lock time are generally performed by lightening parts that move as part of the firing operation, such as the hammer and firing pin or striker, and using a more powerful spring.cite web |url=http://www.davidtubb.com/speedlock.html |title= Speedlock Systems |accessdate=2007-08-31] Further reductions in lock time, to near zero levels, can be achieved with electrical primers.
Bullet dwell time
The bullet dwell time is the time between cartridge ignition, and the time the bullet leaves the barrel.cite web |url=http://findarticles.com/p/articles/mi_m0BTT/is_2000_Annual/ai_61620977 |title=Adjustable Sights |publisher=American Handgunner |accessdate=2007-08-31] Like lock time, dwell time is a window for error, and can be minimized with a faster bullet or a shorter barrel. In some cases a shorter barrel is desired to reduce dwell time, but without losing the sight radius of a longer barrel. In this case a sight extension tube, or "bloop tube" can be used. This is a tube that fits on the muzzle end of the barrel, providing support for the front sight, but that is bored to much larger than bore diameter. This provides the sight plane of a long barrel with less weight and dwell time. [cite web |url=http://www.midwayusa.com/guntecdictionary.exe/showterm?TermID=1330&codeid=140379&path=1.139325 |title=Definition for "bloop tube" |accessdate=2007-08-31]
To ensure consistent, repeatable lockup, tolerances between moving parts must be kept to a minimum. This can be achieved by carefully hand selecting parts, and fitting them together. The best fits are generally attained by selecting slightly oversized parts (or altering stock parts to form an
interference fit) and then lappingthe mating surfaces to attain the desired fit. Tolerances cannot be too tight, however, or functionality will be compromised; this is very important in semi-automatic firearms, where there is a certain amount of energy extracted from the firing of the cartridge that must be used to cycle the action. Overly tight tolerances mean that there is no place for lubricant and dirt to be, and this can bind the parts.
In some cases, stock parts are not sufficient for the task of producing a finished product with sufficiently tight tolerances. In this case, it may be necessary to use specially fabricated parts, built either with minimal tolerances, or parts that are oversized and intended for hand fitting.
The barrel is one of the most important factors for accuracy, as a poorly made barrel may be beyond correction. Even a quality barrel must be well matched to the cartridge it will be firing. In most cases, it is not practical to fix a barrel whose bore is worn out, poorly or unsuitably rifled, or the wrong bore diameter; the primary exception to that is
rimfirebarrels, which can be inexpensively bored out and re-lined with a commercial barrel liner.web cite |url=https://www.brownells.com/aspx/ns/store/productdetail.aspx?p=10974 |title=.17 & .22 BARREL LINER |publisher=Brownell's |accessdate=2007-09-05] If the barrel is unsuitable and relining is not an option, then an aftermarket or custom barrel is the best solution. However, if the bore is good, then there are a number of operations that can be done to the barrel to improve upon its accuracy.
Ideally, the bore must be cylindrical and the rifling geometry precisely the same down the length of the bore.cite web |url=http://riflebarrels.com/articles/barrel_making/details_of_accuracy.htm |title=The Details of Accuracy |author=Jon Haviland |publisher=Lilja Precision Rifle Barrels |accessdate=2007-08-31]
A slight gain in the rifling twist, or a slight tapering down of the bore, will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle.cite web |url=http://ct-precision.com/index2.htm |title=10/22 Gunsmith Notes |publisher=Connecticut Precision Chambering, LLC |accessdate=2007-08-31] cite web |url=http://riflebarrels.com/articles/barrel_making/rifle_barrel_accurate.htm |title=What Makes a Rifle Barrel Accurate? |author=Daniel Lilja |accessdate=2007-08-31]
The twist of the rifling must match the intended ammunition for best accuracy. Rifling with too slow a twist will not stabilize long bullets, causing them to
precessin flight; at its worst, this can result in "keyholing", where bullets strike the target sideways. Too fast a twist can also be a problem, as it can magnify problems in the bullet. A bullet whose center of mass is slightly off-center will diverge at a rate proportional to the rifling twist, so excess twist will result in greater dispersion.cite web |url=http://www.shilen.com/calibersAndTwists.html |title=Products::Rifle Barrels::Calibers and Twists |publisher=Shilen Rifle Barrels |accessdate=2007-09-05]
Barrels also benefit from lapping, not only because it makes the bore a more consistent diameter, but also because it polishes the bore. Barrel lapping should be done with the tool moving in the same direction as the bullet will move, so that any imperfections in the barrel will be smoothed so that they don't interfere with the passage of the bullet. A smooth, polished bore will not only hold the bullet better, but also reduce barrel fouling.
Of great importance to accuracy is the fit of the ammunition to the barrel. The chamber should be concentric, and the throat sized just slightly larger than the bullet diameter. The cartridge must
headspacecorrectly, be held firmly in place, concentric to the bore, and the bullet must fit the bore and be guided to engage the riflingcleanly. Once the bullet is engaged in the rifling, with a good seal and coaxial fit, it must stay that way. It is often possible to shorten a barrel slightly by removing material from the breech end and re-cut the chamber, which can correct many problems in the original chamber.
The crown is the muzzle face of the barrel, and it is the last part of the firearm touched by the bullet before it exits. The crown is also exposed in many firearms, and subject to damage. It is not uncommon for factory crowns to be cut slightly off-center, so that one side of the bullet exits slightly earlier than the other, and this will result in the bullet being pushed away from that side, causing a significant deflection in its path. A crown may be re-cut with relative ease, and this can fix any problems caused by a defective or damaged crown, by ensuring an even release of the bullet.cite book |title=Shooting Times Guide to Accuracy |publisher=Primedia Enthusiast Magazine |author=Shooting Times |authorlink=http://www.shootingtimes.com]
Any machining process on a barrel, whether it's boring, rifling, or turning the outside contour, will produce some stress in the steel of the barrel. This stress can cause the barrel to expand unevenly as it heats, causing shots to "walk" as the barrel heats and cools. To prevent this, careful post-machining heat treatment is often used to stress-relieve the barrels. The amount of good this does depends on the technique used to make the barrel; the
hammer forgingmethod of manufacture, a fast method commonly used by large manufacturers, leaves a significant amount of stress in the barrels, which could be addressed through stress-relieving heat treatment.cite web |url=http://riflebarrels.com/articles/barrel_making/rifle_barrel_accurate.htm |author=Daniel Lilja |title=What Makes a Rifle Barrel Accurate? |accessdate=2007-09-05]
Barrel wear is also an important issue, particularly in high powered firearms. High temperatures tend to erode the barrel at the throat, preventing the bullet from entering the rifling cleanly. One way to produce a long lasting barrel is by the proper selection of materials.
Stainless steels, such as 416, have been shown to have a longer life than the traditional 4140 chrome/ molybdenumsteels used for barrels. While the stainless barrels are not any more accurate than a 4140 barrel, they will maintain their accuracy longer in many applications, as they are more resistant to erosion caused by the heat of firing high powered cartridges. One notable exception to this is the .50 BMGcartridge; competition shooters often fire high precision latheturned bullets made from harder brass, bronze, or steel in this caliber, and the 4140 steel will stand up to this better than stainless.
Another commonly cited accurizing treatment for barrels is cryogenic treatment. This involves slowly cooling the steel down to
liquid nitrogentemperatures, leaving it there for a time, and then slowly warming back to room temperature. This process converts remaining austenitein the steel to martensite. Many proponents of this process claim increased accuracy of the resulting barrels, but independent testing of the process by major maufacturers has shown no increase in accuracy. However, the conversion of austenite to martensite has been shown to result in easier machining and greater wear resistance on steels that tend to have significant amounts of retained austenite, such as stainless steels, and the process does appear to have a significant impact on the accurate life of stainless steel barrels.
A poor fit of the action to the stock is also a source of problems, and this problem is exacerbated by issues such as
thermal expansionof metal parts during use, and swelling and contracting of wooden stocks with changes in humidity. These changes can effect accuracy either by allowing the action to shift under recoil, or by causing slight but accuracy-destroying bending of the barrel. By removing wood from areas of contact and, when needed, replacing it with a more stable substance that is precisely fitted, such as a fiberglass composite molded in place, a more stable, precise fit can be achieved.cite web | url=http://hunting.about.com/cs/guns/a/float_bed.htm |title=Floating and Bedding: Accurizing Your Rifle|accessdate=2007-08-30] Other materials, such as composites or laminated wood, can also provide a stronger, more dimensionally stables stock than traditional woods can. Some stocks are even being made out of aluminum or other metals, for maximum stability.
The process of fitting the action to the stock is called "bedding", and there are a number of different processes used. "Glass bedding" uses a fiberglass composite which is molded in place around the action. This can be as simple as bedding the recoil lug of a
bolt actionrifle, or as complex as bedding the entire action and barrel channel. "Pillar bedding" uses precisely machined metal pillars in the stock which mate with the action, providing a strong metal-to-metal contact without the labor intensive hand fitting required in glass bedding.
Many times it is advantageous to eliminate most or all contact between the stock and barrel, to eliminate potential accuracy-destroying interference in the barrel harmonics. To do this, stock material is removed along the barrel channel to leave a slight gap between the stock and the barrel; this is called "free floating" the barrel. With some designs, "pressure bedding" is preferred, where a single point of contact remains between stock and barrel near the forend. In both these cases, bedding the action is required to provide support for the floating or pressure bedded barrel. Since the action ends up supporting the mass of the barrel, and a poor stock to action fit will result in unacceptable shifting.
The primary purposes of the firearm action are holding the cartridge in place in the chamber, and providing a way to ignite the propellant. In a
single-shotaction, little additional functionality is provided, while in a semi-automatic firearmthe action also taps energy from the firing process for cycling to fire the next round. From an accuracy perspective, the primary goal of the action is to achieve a consistent placement of the cartridge in the chamber every shot.
The term blueprinting, borrowed from high performance engine builders, is also often applied to the process of manufacturing or altering parts to a tighter tolerance than stock parts. Blueprinting a firearm action involves similar operations designed to tighten the tolerances of a firearm action to ensure consistent and correct seating of the cartridge in the chamber. A typical set of blueprinting operations for a bolt action firearm would include the following:
#Bolt blueprinting operations:
##Squaring the bolt face
##Ensuring the bolt is concentric in the action
##Squaring and lapping the locking lugs
##Sleeving the bolt, then machining down for a precise fit to the action
#Receiver blueprinting operations:
##Making the threads for the barrel concentric
##Making sure the breech face is square to the action centerline
##Making sure the recoil lug is square to the action
##Squaring and lapping the locking lug recesses
Revolver specific issues
The defining characteristic of a
revolveris the revolving cylinder, separate from the barrel, that contains the chambers. Revolvers typically have 5 to 9 chambers, and the first issue is ensuring consistency among the chambers, because if they aren't consistent then the point of impact will vary from chamber to chamber. The chambers must also align consistently with the barrel, so the bullet enters the barrel the same way from each chamber.cite web |url=http://www.jesseshunting.com/articles/guns/category8/167.html |title=Making a Case for the Revolver |publisher=Jesse's Hunting and Outdoors |accessdate=2007-09-26]
The throat in a revolver is part of the cylinder, and like any other chamber, the throat should be sized so that it is concentric to the chamber and very slightly over the bullet diameter. At the end of the throat, however, things change. First, the throat in a revolver is at least as long as the maximum overall length of the cartridge; if otherwise the cylinder cannot revolve. The next step is the cylinder gap, the space between the cylinder and barrel. This must be wide enough to allow free rotation of the cylinder even when it becomes fouled with powder residue, but not so large that excess gas can be released. The next step is the forcing cone. The forcing cone is where the bullet is guided from the cylinder into the bore of the barrel. It should be concentric with the bore, and deep enough to force the bullet into the bore without significant deformation. Unlike rifles, where the threaded portion of the barrel is in the chamber, revolver barrels threads surround the breech end of the bore, and it is possible that the bore will be compressed when the barrel is screwed into the frame. Cutting a longer forcing cone can relieve this "choke" point, as can lapping of the barrel after it is fitted to the frame.cite web |url=http://www.alphaprecisioninc.com/revolver/default.htm |title=Revolver Accuracy by Alpha Precision, Inc |accessdate=2007-09-26] cite web |url=http://www.chuckhawks.com/used_handgun.htm |title=Buying a Used Handgun |author=Chuck Hawks |accessdate=2007-09-26]
A consistent lockup is important to keep all these parts in line, and revolvers are prone to abuse that can damage these parts, adversely affecting the accuracy and even safety of the revolver. This lockup consists of two parts, the crane to frame lockup, and the cylinder bolt to cylinder lockup. Many swing-out cylinder revolvers only support the cylinder securely at the rear, and flipping the cylinder open and closed can bend the crane and prevent the cylinder from lining up parallel to the bore. The cylinder bolt, which engages the bottom of the cylinder through a slot in the frame, should provide a relatively tight lockup, and not drag the cylinder during rotation or pop loose when the hammer is cocked at a reasonable speed. Fanning a revolver can batter the cylinder bolt and prevent a solid lockup.
During firing, the chamber pressure rises from atmospheric pressure to, in a typical rifle cartridge, pressures of about 50,000 psi in a milliseconds. This rapid increase in pressure causes the barrel to vibrate at a certain natural
frequency, much like a tuning fork. The point in time at which the bullet exits the barrel will determine the orientation of the muzzle relative to its rest position. Exiting near a peak or valley in the motion means the muzzle is relatively stationary, and shot dispersion will be minimized; exiting between a peak and valley means the muzzle is rapidly moving, and shot dispersion will be greater.cite web |url=http://www.varmintal.com/atune.htm |title=Barrel Tuner Vibration Analysis: Effects of tuner adjustments on vibration frequency and the shift in barrel node points. |accessdate=2007-08-31]
There are two ways to address harmonics; reducing the amplitude with a stiffer barrel, or working with the natural frequency to minimize dispersion.
Stiffness of a barrel is proportional to the fourth power of the diameter, and inversely proportional to the third power of the length. Because of this, short, thick barrels will vibrate with high frequency and low amplitude, and long, thin barrels will vibrate with a low frequency and high amplitude. Due to the impact of length, barrel harmonics are primarily a concern with rifles. By using the shortest and/or fattest barrel possible, the amplitude of the vibrations can be minimized to the point that they are irrelevant to accuracy. Unlimited class
benchrest shootingbarrels, where weight is of very little consequence, have very large diameters; an outside diameter of 2 inches (5 cm) is not uncommon.cite web |url=http://riflebarrels.com/articles/barrel_making/rigidity_benchrest_rifles.htm |author=Daniel Lilja |title=A Look at the Rigidity of Benchrest Barrels |accessdate=2007-08-31]
While standard rifle barrels taper from breech to muzzle, high precision rifles will often use a barrel with far less taper, called a "heavy barrel", sometimes leaving the barrel cylindrical all the way to the muzzle, called a "bull barrel". Either technique greatly increases the stiffness of the barrel by enlarging the average diameter, but this process adds significant weight as well. This can greatly increase the mass of the barrel, however; going from a lightweight sporter contour to a heavy barrel contour can double the mass, and a going to a bull barrel contour can more than triple it. Fluting, consisting of grooves machined in the outer surface of the barrel to remove material, can reduce the weight while maintaining most of the stiffness.cite web |url=http://www.riflebarrels.com/products/software.htm |title=Contoured Barrel Weight Calculation Program |publisher=Lilja Precision Rifle Barrels |accessdate=2007-09-06]
Barrel tensioning devices are a way of gaining stiffness with minimal increases in weight. They do this by using placing a lightweight sleeve, often made of aluminium or a
carbon fibercomposite, around the barrel, and then using a nut attached to the end of the barrel to tension the barrel and place the sleeve under compression. This serves to keep the muzzle closer to concentric and coaxial to the breech during vibration.cite web |url=http://accuracysystemsinc.com/ruger_mini_14_30.html |title=Barrel tuners for the Mini-14|accessdate=2007-08-30]
The other solution is to work with the barrel's natural vibration, and tune the components so that the bullet exits the barrel as it is moving the slowest. The simplest approach to harmonic tuning is to concentrate on the ammunition. The
internal ballisticsof a given cartridge will determine its "dwell time", or the time it takes from ignition to exiting the barrel. By experimentally matching the dwell time to the barrel's frequency, the best load for a particular firearm may be found. Similarly, handloadinggives the shooter the opportunity to very precisely control the bullet velocity, and experimentally choose the optimum velocity.
If it is not possible or desirable to match the bullet to the barrel, there are a number of devices marketed to allow the barrel to be tuned to match the ammunition. There are a number of models of these that work in different ways. One type uses an adjustable damper or pressure bedding point to allow the shooter to find the "sweet spot", where it will do the most good at damping the vibrations that are affecting accuracy.cite web |url=http://rifle-accuracy.com/gmag1.htm |title=Article on AccuMajic Accurizer] |accessdate=2007-08-30] cite web |url=http://www.limbsaver.com/Products/Firearms/Barrel_De_Resonator.aspx
title=Barrel De-Resonator |publisher=Limbsaver |accessdate=2007-08-31] Other tuners work by using an adjustabe weight on the muzzle to alter the length of the resonant portion of the barrel, and allowing the frequency to be matched to the ammunition.cite web |url=http://www.browning.com/faq/detail.asp?ID=107 |title=How does the BOSS system work? |publisher=Browning |accessdate=2007-08-31] [cite web |url=http://www.browning.com/faq/detail.asp?ID=109 |title=I have a Browning rifle with the BOSS. What is the "Sweet Spot" setting for my firearm? |publisher=Browning |accessdate=2007-08-31]
The difference between an airgun and a firearm is the way in which the power to launch the projectile is provided. In a firearm, it is provided by a chemical reaction, and in an airgun is it provided primarily by a the expansion of a compressed gas, typically either air or
carbon dioxide(CO2). These gases are used primarily for convenience, some airgun variants run on other gases, such as refrigerants like R-134a commonly used in airsoftguns, or hydrogenused in light gas guns.
There are three primary types of powerplant used in airguns:
*Pneumatic, which uses pre-compressed air
*Piston, which uses a piston to compress air at the moment of firing
*CO2, which uses a reservoir of liquid carbon dioxideEach method has its own advantages and disadvantages, and different areas that can be addressed to ensure consistency.cite web |url=http://www.airguns.net/general_airgun_types.php |title=The Three Basic Types of Airguns |author=Ben Saltzman |publisher=American Airguns |accessdate=2007-09-13]
With recent advances in airgun technology, the most powerful systems will produce velocities near or exceeding the
speed of soundwith lightweight pellets; this, however, is not a good thing where accuracy is concerned. Pellets have a poor ballistic coefficient, and quickly lose velocity; when they drop below the speed of sound, they will often tumble. However, high velocities sell airguns; if accuracy is desired from these high velocity guns, then heavier pellets should be used to keep the velocity down. This will provide not only better accuracy, but better downrange retention of velocity and power.cite web |url=http://www.airguns.net/general_inaccuracy.php |title=Inaccuracy in Airguns |author=Lewis Reinhold |accessdate=2007-09-13]
Pneumatic systems used compressed gas for power, usually compressed air. This air may be compressed by the gun for each shot, in a "single stroke" or "pump" (multiple stroke) gun, or it may be "precharged" by an external compressor.
A single stroke system, as the name implies, uses a single stroke of the pump to compress a cylinder full of air, which is then all used for one shot of the gun. Single stroke systems are both inexpensive and capable of high accuracy due to the simplicity and consistency of the single stroke design. The downsides are the low power provided, though this is not a handicap in standard 10 meter airgun target shooting. When Daisy introduced an inexpensive single stroke pistol, the model 717, in the late 1970s, American Olympic shooter Don Nygord demonstrated the potential of the design by shooting an accurized version in a California State air pistol championship, and winning the gold medal. The (at the time) US$40 Daisy, with a better rear sight and an adjustable trigger added, shot as well as the US$400 Olympic class air pistols it was competing against. [cite journal |title=How to Make a Super Shooter: How to Accurize Your Daisy 717 For Practically Nothing and Outshoot the High Priced Competition |last=Nygord |first=Don |year=1980 |month=April |journal=American Marksman]
More powerful is the pump system, which is a slightly more complex version of the single stroke design. Rather than leaving the air in the piston when compressed, the pump airgun has a reservoir to contain the compressed air, allowing multiple pumps to be used, typically 2 at a minimum, up to 10 pumps for full power. The ability to vary the power, however, it's the pump airgun's major disadvantage when it comes to accuracy, as it makes it very difficult to get a consistent charge. The increased power of a pump airgun makes it an attractive choice for many shooters, and there are steps that can be taken to improve consistency, such as modifying the air chamber so that not all the air is exhausted in a shot.cite web |url=http://www.mac1airgun.com/steriodags.html |title="Steroid Modified" BENJAMIN/SHERIDAN |accessdate=2007-09-13]
The last type of pneumatic airgun is the precharged pneumatic. This is both an old and a new design; some of the earliest airguns, such as the model carried by
Lewis and Clark, were of this type, as are many new cutting edge models. The precharged pneumatic uses an external source of compressed air, either an external pump or a high pressure reservoir such as a SCUBAtank, to fill a reservoir. The reservoir can be a small single shot one, such as in the Brocock Air Cartridge system, or a large, multi-shot tank. The key to top accuracy in a precharged pneumatic is a consistent pressure. With multishot systems (as are most), the pressure in the reservoir will drop with each shot fired, so the best way to achieve consistency is with a pressure regulator, which provides a steady, but lower, pressure at the valve, for as long as the reservoir pressure remains higher than the regulated pressure. Regulators are also generally adjustable, so a low pressure setting will provide many shots of lower power, while a high pressure setting will provide a few high power shots.cite web |url=http://www.airguns.net/general_regulators.php |title=Airgun Regulator Basics |accessdate=2007-09-13]
Piston airguns, often called "springers", are unique in many ways. Since the firing process involves a fairly massive piston suddenly moving to compress the air, they have a significant "kick", generally called "recoil" (though this is not the same as firearm recoil). The recoil begins when the piston starts to move forwards, which pushes the rest of the gun backwards. Then recoil then stops suddenly as the piston reaches the end of its travel, and is brought to a stop by the cushion of high pressure air trapped between the piston and pellet. This recoil can be brutal on the gun in high powered models, and will loosen screws, shift sights, and break scopes not designed specifically for the unique recoil of piston airgun--all of these can lead to poor accuracy. In addition to the recoil, piston airguns have a long lock time, as the piston must compress the air before the pellet begins to move, and the gun is moving due to the recoil during this time. Spring airguns require a special technique to fire, to ensure that the gun moves very consistently during this recoil. The preferred method is a very loose hold, to allow the gun to move back; this means that a piston airgun will "not" shoot the same from a bench. All accuracy testing and sighting in must be done in the same position the gun will be shot from, otherwise the results will be different. Piston airgun recoil cannot readily be dealt with without a significant redesign; in some cases the action can be mounted on a sliding rail, or two counter-recoiling pistons can be used, but this requires significant changes to the design. The resulting gun will be far less sensitive to the shooter's hold, and thus much easier to shoot accurately.cite web |url=http://www.pyramydair.com/cgi-bin/model.pl?model_id=398 |title=Diana RWS-45 |accessdate=2007-09-13 The RSW-54 is a model 48/52 with the action mounted on rails, so it can recoil within the stock.]
The first step to accuracy of a piston gun is to ensure that all screws are secure and the sights are rated for use on a piston airgun. Another potential issue regarding accuracy is resonance in the spring used to power the piston in most airguns. The spring will vibrate strongly as the piston stops, and this will impact the harmonics of the gun. A
gas springwill, if one can be fitted to a given model, provide vibrationless action, though with some loss of efficiency and even sharper recoil.cite web |url=http://www.pyramydair.com/blog/2006/04/gas-springs-of-theoben-airguns.html |title=Gas springs of the Theoben airguns! |accessdate=2007-09-13] Spring driven pistons also respond well to accurizing; careful fitting of parts and use of quality lubricants and spring damping "tar" can reduce the level of vibrations and improve accuracycite web |url=http://www.pyramydair.com/blog/2006/08/spring-gun-tune-part-13-range-testing.html |title= Spring gun tune: Part 13 Range-testing the R1 we tuned |accessdate=2007-09-13 Contains links to all sections of article on the tuning of an R1 air rifle]
CO2 is commonly found in multishot airguns, from the cheapest plinkers to Olympic class target guns, though the latter are facing competition from regulated precharged pneumatics. CO2's advantage is that it is stored in a liquid form, rather than a gas, and as such provides a greater power density. The liquid also provides a constant pressure, the
vapor pressure, as long as there is liquid remaining in the reservoir. The downside to CO2 is that it is dependent upon the vapor pressure, which changes significantly with temperature. This is of primary concern to outdoor shooters, who may shoot in widely varying temperatures, or for rapid fire shooters, as rapid release of the gas results in a rapid drop in the temperature of the liquid.
The issue of temperature change is not readily addressable, other than by the use of readily adjustable sights, so the shooter can adjust the sights to match the point of impact based on the current ambient conditions. In the case of rapid fire, there is a solution that can provide far greater stability for many guns. The original CO2 airguns were filled from an external source of CO2, but in 1954
Crosmanintroduced the 12 gram Powerlet, a compact, disposable tank now ubiquitous in inexpensive CO2 airguns. The downside to these is that the small quantity of CO2 liquid cools down quickly, leading to a rapid drop in velocity and a changing point of impact. By moving to a bulk fill system, with a much larger tank, there is more liquid available, and the greater mass will cool far more slowly.cite web |url=http://www.mac1airgun.com/ldpistol.html |title=The Mac1 LD Air Pistol |accessdate=2007-09-13]
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