Accurizing is the process of improving the accuracy of a firearm. The goal of accurizing is consistency. If every part of the process of firing can be made as consistent as possible, then the bullet should strike the target as close as possible to the same place every time. Since most modern firearms are mass produced, and since mass production and interchangeable parts all have some allowance for error, most firearms can gain significant accuracy with some additional work.
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.
- 1 Defining accuracy
- 2 Usability
- 3 Tolerances
- 4 Harmonics
- 5 External links
While the question "which of these guns is the most accurate" seems to be a fairly simple one to answer—just shoot and measure—defining "what is accurate" is not necessarily an easy task. For a hunting gun, for example, the first shot fired, from a cold, clean barrel, is often the most important. Target shooters, on the other hand, care as much about the last shot as the first, and so will actually fire fouling shots before shooting for score, to get the barrel warmed up and dirty so the first shots on the target will have the same conditions as the last. There is also the issue of ammunition—some guns may be very picky about ammunition, going from excellent groups with one ammunition to horrible groups with another, while others will shoot just about anything with the same performance. Depending on the shooter and the situation, a gun may be chosen based on its best case performance, its average performance, or even its worst case performance.
A perfect example of the ambiguity in the "which is most accurate" is a test run by Performance Shooter magazine in December, 1996. The magazine was testing seven brands of 38 Special wadcutter rounds in three different revolvers, a Smith & Wesson Model 686 and Model 52, and a Colt Python Target 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).
Based on average group 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 deviation of 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 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.
The keys to 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. For firing multiple shots, it is also best to minimize the amount of movement that must be made between shots, so that the ideal grip, once obtained, can be retained as easily as possible.
The enemies to usability are fatigue and anticipation of recoil or report. Fatigue causes muscles to tremor, eyes to lose focus, and concentration to lapse. Anticipation of recoil or report causes the user to jerk the trigger and to tense at the moment of firing, which will throw the gun out of alignment and cause a miss.
Eliminating grip fatigue
Any portion of the gun that must be gripped should be the right size for the shooter's hands. Too large and the user must grip too tightly, causing tremors and fatigue; too small, and a firm, consistent grip becomes very difficult to achieve. Ideally, the rifle stock or pistol grips should be shaped to fit the shooter's hand, so the gun in effect guides the shooter's hand into the same place each time. The hand used to squeeze the trigger is the most important; swells located under the palm of the hand, a shelf for the thumb to rest on, and a shelf to separate the trigger finger and guide it into place will all help. Some shooters prefer a smooth grip surface, especially on rifles, while others prefer a rough, checkered or stippled surface on some or all of the grip area. The angle of the grip is also important. For rifles the grip angle should ideally keep the wrists straight and the elbows down. For pistols, low recoiling calibers like .22 Long Rifle generally often have a grip angle of about 30 degrees from vertical, while heavier recoiling calibers have a straighter angle, around 20 degrees. The higher grip angles encourage a relaxed wrist, while the steeper angles work better with a locked wrist and a firmer grip.
While pistols only have significant contact with the trigger hand, rifles generally also have contact with the shooter's other hand, cheek, and shoulder—the exception being guns that are designed to be fired from a sandbag or other rest, which are often used with just cheek and trigger hand. The forend, where the non-trigger hand grips a rifle, is the main source of control in the aiming of the rifle. It should be wide enough to fill the hand without the fingers touching the barrel (which can get quite hot) and long enough that the user can pick a comfortable elbow angle for any shooting positions used with the rifle. Since the supporting hand may be placed anywhere along the forend, forends tend to have a constant cross-section along the useful portion of their length.
The weight of the gun is also an important consideration. A gun that is too heavy will result in a tighter grip and quick fatigue due to the effort of holding it. Too light can also be a disadvantage, since a heavy gun has more inertia and will be less subject to involuntary muscle movements.
Eliminating eye fatigue
Sights should be suited to the targets used. For large targets engaged at high speeds, large sights are best, for small targets requiring high precision, a finer sight would be best. Since the eye is capable of focusing only at one distance at a time, the sights should be selected so that the point of focus is where it will do the greatest good, which is usually the front sight. An aperture rear sight is best (where allowed by rules and other considerations) because even when out of focus, it is easy to center the target in the resulting fuzzy circle. On target rifles, circular front sights are often used as well; the target bullseye is centered in the front circle, which is then centered in the rear circle.
Pistol sights are rarely of the aperture type; they are most often Patridge style sights. When selecting a Patridge sight, the front sight blade should be wide enough to focus on easily, and the image of the target should sit centered on top of the sight blade. The rear sight should be chosen so that a good amount of light is visible on both sides of the front blade. Since the rear sight is always out of focus, the intensity of the light bars between rear sight and front sight are used to center the front sight—when both sides are equally bright and the top of the blade is even with the top of the rear sight, then the sights are correctly aligned.
Optical sights (usually called scopes) are the least fatiguing form of sight, and the easiest to use. The optical sight visually projects a single aiming point (typically a crosshair) to the same plane as the target image. Often the target image is magnified, but this is not necessary; many pistol scopes, especially those used in action shooting, have no magnification. Magnification has tradeoffs in field of view; if you double the target image magnification, you cut the angle that can be viewed in half. Magnification also magnifies the shooter's movements—a hold that looks perfectly steady with iron sights can be seen to wander around the bullseye when seen through a high magnification scope. This can be good or bad, depending on the shooter. A good shooter will use the magnification to increase the steadiness of their hold, while a poor shooter will be distracted by the movement and try to jerk the trigger as the crosshair slides across the target.
Since the rifle acts as a rest for the shooter's head, it is important that the stock provide both a secure rest for the cheek and a good placement for the shooter's eye. A rifle with optical sights, for example, should have a higher cheek rest than a rifle with non-optical sights, since optical sights tend to be higher. Cheek position also can impact accuracy. Optical sights, since they project the crosshair out to a given distance, suffer from parallax errors at other distances. As long as the crosshair is centered in the lens this will have no impact. However, if the crosshair is viewed off center parallax error will come into effect.
Optimizing the trigger pull
The trigger must fulfill two goals: it must fire the round with the minimum of disturbance to the shooter's grip, and it must not fire the round until the shooter is ready to fire. The force required to pull the trigger, called the trigger pull, and the consistency of the pull are both important for top accuracy.
For some guns, such as handguns used for combat pistol shooting, it is reasonable to expect that the gun may be dropped, so the trigger should be held forward with sufficient force to prevent it from being pulled by the force of a fall. For a heavy benchrest gun, that is intended only to be fired from a shooting bench and is never loaded except when on the bench and ready to fire, a much lighter force is allowable. Unfortunately, many manufacturers are afraid of potential lawsuits caused by negligent discharges, and so they build firearms with far heavier trigger pulls than is needed for top accuracy under normal usage conditions. This can almost always be fixed by using either aftermarket replacement parts, or by careful hand tuning by a competent gunsmith. Trigger pulls on competition guns range from 10 pounds force (44 newtons) on a double action revolver to as low as 2 ounces (0.6 N) on a benchrest rifle.
Consistency is always important, because sudden changes in the amount of force required to pull the trigger will result in sudden increases or decreases in muscle tension, which can alter the grip on the firearm. Even after the trigger breaks, or begins the process of firing the round, it is important to maintain a steady force. There is a short but significant lag between the trigger break and the firing pin striking the primer, called the lock time, and any movement during the lock time can still disturb the path of the bullet.
Many target guns will have triggers with one or more of the different aspects of the pull fully adjustable. Anschutz, for example, makes triggers where takeup distance, first stage weight, second stage weight, sear engagement, and overtravel are all independently adjustable.
Many triggers, especially those in semi-automatic guns, will have some slack that needs to be taken in before the trigger engages the sear and begins the process of firing the gun. This is especially prevalent in semi-automatics because the trigger has to move forward after firing to re-engage the sear, which will have been disconnected during the cycling of the action. This takeup should be fairly short, so that the shooter's finger doesn't shift significantly on the trigger during the takeup movement. Any shifting will result in a less than ideal angle between trigger and finger, and this can cause the gun to be pushed to the side or elevated as the trigger is pulled further.
The release is the portion of the trigger travel where the sear is being disengaged. Lower tension on the sear and reduced sear travel will both make the release a smooth, "crisp" operation (crisp meaning that there is little or no noticeable travel during release). Too much reduction in sear engagement, and the firearm becomes subject to unintentional discharge; reducing the tension means reducing the hammer or striker spring tension, and that can lead to failure to detonate the primer and excessive lock times. Another problem is getting the weight of the release so low that the inertia of the trigger will release the sear if the firearm is jarred.
One way to get a very light release without risking unintentional discharged is to use a two-stage trigger. A single-stage trigger has little or no takeup; if there is takeup, it is purely due to slack in the trigger mechanism. A two-stage trigger is designed to have a distinct takeup, with the trigger working against a significant amount of spring pressure. At the end of the takeup travel, there is a small but noticeable increase in pull as the sear begins to disengage. The two-stage trigger might have a pull force of several pounds force, but first and second stages differ in pull by only ounces. As the shooter prepares to fire, enough pressure is applied to handle the takeup, or first stage. Then the shooter pauses to finish aiming, and then the tiny amount of additional pressure is applied to overcome the release, or second stage of the pull. This gives the handling safety of a heavier trigger pull, with most of the advantages of a light, crisp single-stage trigger.
Overtravel is the travel made by the trigger after the sear is released. Generally, when the sear releases, there is a noticeable reduction in the force required to move the trigger. Since the shooter has already applied enough force to release the sear, the sudden reduction in pressure causes the trigger to move back rapidly—far more rapidly than the shooter can react to the change. Since this happens at the most critical moment of firing, and happens too fast for the shooter to react, this is the most difficult aspect of the trigger pull for the shooter to overcome through technique alone. The best way of dealing with overtravel is to not allow it. An adjustable stop placed just past the point of sear release will stop the movement of the trigger, preventing the sudden release in tension and the subsequent movement. This stop needs to be adjustable, since normal wear on the sear and trigger components will change the point of release slightly, and the overtravel stop will need to be adjusted periodically to compensate. Overtravel is most critical on pistols, where the inherent instability of the shooter's grip combined with the heavier trigger pulls encountered can result in deviations measurable in degrees of arc.
Other usability issues
The location of the various controls for a firearm can have a significant effect on accuracy. The most vital is, as one would expect, the trigger. The trigger should be easy to reach, so the user doesn't have to stretch or strain to reach it. The trigger placement should also cause the finger to naturally lie along the trigger in the best position to pull it—for most guns that would be with the trigger centered in the pad of the tip of the trigger finger. The exception to this is double action triggers, where most people shoot best with the trigger at the first joint of the finger.
Other controls can have a positive or negative on the user's comfort and consistency. Any controls that interfere with the user's grip, or cause discomfort due to poor positioning can cause the user to lose concentration or rush a shot, leading to a miss. A well located safety can help a shot in cases, such as hunting, where the firearm is carried with the safety on. A well designed breech, whether it be bolt action, lever action, or even a single shot design, can speed up reloading time, giving more time to concentrate on aiming a successive shot. Semiautomatic]designs should eject fired cases away from the shooter—for example, nothing destroys the concentration quite like a hot 10mm Auto case down the back of one's shirt.
Another issue that is not strictly related to the gun design, but does certainly apply to fitting the gun to the user is recoil. A heavy recoiling caliber can hurt to shoot. If the shooter is not comfortable with the recoil of the gun and cartridge combination they are firing, they will anticipate the pain, which can cause them to flinch before the shot is fired. This is a very common problem, and can be demonstrated by the classic "ball and dummy" drill, where live and dummy ammunition are mixed together, so that the shooter doesn't know ahead of time if a given pull of the trigger will fire the gun or not. If there is any movement of the gun when a dummy is fired, it indicates the user is anticipating the recoil and would have pulled the shot off target. If, even when focusing on good technique, a shooter still flinches on the dummy shots, then it is time to look at methods for reducing felt recoil. A padded grip or buttplate, a muzzle brake, a heavier gun, a lighter cartridge, or a combination of those items can go a long way to reducing the felt recoil, improving the user's confidence, and eliminating the anticipation.
Tolerances between moving parts are very important when it comes to maintaining a high level of consistency. Loose parts will tend not to fit together the same way every shot, and those differences can result in poor accuracy. This does not mean that tolerances should be as tight as possible, however. Firearms are generally used outdoors, and even modern smokeless powder leaves some residual soot in the gun. Dust, moisture, and soot will build up on the moving parts, and if there isn't sufficient allowance for some buildup then parts that should lock into place won't lock anymore, and parts that should slide smoothly will become stiff. As Mickhail Kalashikov, designer of the AK-47, said in reference to his creation, "Best is the enemy of good enough". The AK-47 is a military rifle, designed to be used by often poorly trained, conscripted troops, in all weather and all seasons. By allowing plenty of room around the moving parts, Kalashnikov created a rifle that on its best day might shoot under 5 minutes of arc, but will shoot under conditions that will cause most other firearms to fail miserably. The M16, on the other hand, is often criticized for unreliability under extreme circumstances, but most are capable of 2 minutes of arc or better, with match grade examples shooting along with some of the most accurate rifles in the world, with samples often producing groups of 0.25 minute of arc at ranges of under 200 yards. Picking the right balance between accuracy and reliability depends entirely on the situation in which the gun will be used. For a gun used only for shooting targets, the preference will be to trade reliability for accuracy; for a defensive weapon, reliability is far more important. For a sport hunting rifle, the balance will be somewhere in the middle.
Sight to barrel alignment
One of the most obvious areas where tolerances affect accuracy, especially in semiautomatic pistols, is the sight-to-barrel fit. Since one or both of the sights are often located on the slide of the pistol, which moves independently from the barrel, having the slide and barrel lock up consistently is very important. Since free and easy movement of the slide is also important for reliability, production guns often err on the side of a loose tolerance. Aftermarket barrels are often machined slightly oversized, so that careful removal of metal is needed in areas critical to the lockup. This allows the gunsmith to carefully match the barrel to the slide, providing as tight a lockup as desired for the proper balance of accuracy and reliability. Scoped pistols add an additional level of difficulty, as the scopes are often mounted to the frame, which is independent of both slide and barrel in the common short recoil design. In this case, the slide must be fitted to the frame, then the barrel to the slide. It is for this reason that competition quality pistols are often 2 or 3 times the price of the mass-produced versions.
Sight-to-barrel alignment also applies to some rifles, particularly the Ruger 10/22. Since the barrel slips into the receiver rather than threading in, the junction is relatively flexible, especially when the barrel heats and expands, pushing against the wooden stock. Since scopes are generally mounted on the receiver, special care must be taken to minimize the effects that barrel heating can have on this joint. Another approach is to use a cantilever mount, which mounts directly to the barrel, and has a bar that extends over the receiver. This bar may bolt to the receiver, adding strength to the joint, or may float entirely above it, with the scope mounted to the bar rather than the receiver.
Bolt to barrel fit
When a cartridge is fired, the pressures in the case can range from 10,000 to 65,000 psi (70–450 MPa, 700–4,500 bar) for common cartridges. The case, usually made of brass, expands under the pressure of firing to fit the chamber tightly. If the case is loose in the chamber before firing, this will cause a potentially significant change in the chamber volume, which will have an impact on the pressure curve. The sudden pressure can also cause a loose bolt to shift back, which will also change the effective chamber volume and thus the pressure curve. The initial shape of the pressure curve can have a profound effect on the behavior of the cartridge. For example, the 357 SIG cartridge, which has a very short neck to hold the bullet in place, has been observed to show significant case deformation due to serious overpressure by the bullet being pushed just 1/8" too deep in the case. This shows how much pressures can change with even a small reduction in effective chamber volume; while the effects of an increase aren't nearly as dangerous, they can cause accuracy problems.
To achieve the best possible bolt-to-barrel fit in a bolt action rifle, lapping compound is applied to the locking lugs of the bolt, then the bolt is locked in place with a dummy cartridge in the chamber. The dummy case pushes the bolt back, causing the lugs to lock up firmly. The lapping compound will quickly smooth out any high spots, and provide a smooth, solid surface for the bolt to lock into. For best results when firing the rifle, cases that have previously been fired in the rifle are reloaded, and are resized only at the neck, so the case will fit into the chamber as tightly as possible. This prevents any space at the shoulder of the chamber for the case to expand into. This solution is generally only viable for bolt action rifle and pistols, since the bolt action provides enough leverage to push the tight fitting cartridge into the chamber. While semiautomatic and other designs can benefit from a good bolt to barrel fit, they do not have the power to cam a neck-sized cartridge into place, and so full length resized cases must be used.
Movement under recoil
Upon firing, a significant amount of momentum is transferred to the firearm in reaction to the acceleration of the bullet. While this may only accelerate the rifle to a speed measurable in inches per second, that force is applied in milliseconds. This sudden acceleration is eventually transferred to the user, whose flexible body absorbs and dissipates the energy. Even transfer of this momentum must be consistent to achieve the best accuracy, because the momentum transfer is well under way before the bullet exits the barrel.
With rifles, the big issue is the fit between stock and barrel. The stock is a significant part of the mass of the rifle, and if it shifts relative to the barrel and/or action, then it changes how the rifle reacts to the recoil. This concern is addressed by bedding the action. When an action is bedded, portions of the action and the corresponding portions of the stock are made to fit together very precisely. In pillar bedding, this is done by machining metal (usually aluminum) pillars that perfectly match mating spots on the barreled action. The pillars are then epoxied into the stock, and the barreled action screwed down tightly to the pillars. The other method, far more accessible to the do-it-yourself user, is glass bedding. In this method, material is removed from the stock around the critical areas (generally where the screws attach action to stock) and then these sections are dammed off with a soft substance like clay. These areas are then filled with a mix of epoxy and fiberglass or other high strength filler. The barreled action, covered in a suitable mold release agent, is then screwed into the action and left while the epoxy cures. If all goes well, the result, after cleanup, will be a stock that contains a perfect impression of the barreled action - of course, if all doesn't go well, the user ends up with the action glued to the stock, which generally means destroying the stock to remove the action. Some die-hard accuracy shooters who consider a barrel with more than 1,000 rounds through it "worn out" will go so far as to intentionally glue the barreled action to the stock, with the intent of tearing it up when the barrel must be replaced. This makes it impossible to disassemble the rifle without destroying components, so most shooters are not willing to go to that extreme.
To explain the importance of movement under recoil in pistols, first consider the following paradox. Take a given pistol and cartridge combination, say a 45 Colt firing a 200 grain (13 g) bullet at 900 feet per second (270 m/s). After adjusting the sights so the pistol hits point of aim at 50 feet (15 m) with that load, switch to a 350 grain (23 g) bullet at 750 feet per second (230 m/s). Where does the round impact relative to the point of aim with the new cartridge? Logic says that the slower bullet will drop more during its longer flight, and hit lower than the lighter, faster bullet. In reality, it is far more likely that the slow, heavy bullet will hit higher than the fast light bullet. The reason for this is that the slow, heavy bullet produces about 50% more recoil than the light fast bullet. Since the barrel sits far above the pistol's center of mass, the recoil will torque the pistol so as to raise angle of the barrel. Unless the pistol is clamped firmly into a vise, there will be insufficient force to resist this torque. The phenomenon of "limp wristing" a pistol, that is, holding it so loosely that the cycling of the action will fail, is widely observed and well documented. While not as obvious, the firearm begins to shift in the user's grip before the bullet exits the barrel, and so can impact accuracy. Choosing a grip that allows the user to firmly hold the gun, especially against the rotation of recoil, can help ensure consistency.
Bullet-to-barrel fit is, as one would expect, very important to accurate shooting. A bullet that does not fit tightly in the bore will let gas escape, causing erratic velocities, and will tumble in the bore so that it exits at an angle, which is likely to cause tumbling. In addition, the shape of the bore and the rifling can have quite an impact as well.
While the barrel of a firearm may seem very stiff and resistant to significant vibrations, the force of an igniting cartridge, which goes from zero pressure to 10,000 to 60,000 lbf/in² (70–400 MPa, 700–4,000 bar) or more in less than a millisecond, creates enough force to make the barrel ring like a tuning fork at its natural frequency. It has been experimentally shown that such ringing can open groups up several minutes of arc, especially with long, thin barrels that ring at lower frequencies and higher amplitudes than short, thick barrels. Harmonics don't really have an impact with handguns, since the barrels are far shorter and thus inherently far stiffer than rifle barrels. Most target rifles have larger diameter barrels, appropriately called "heavy barrels"; some go so far as to have no taper at all along their length, and these are called "bull barrels". The large diameter barrels reduce the amplitude very significantly, but at the expense of a lot of weight. Sporting rifles and rifles made for target matches that have a maximum rifle weight cannot afford to have a barrel that heavy, and legal and internal ballistics considerations set a minimum practical length. For these rifles, alternate methods of dealing with harmonics must be used.
No method can completely eliminate the effects of harmonics, and because of that, an accurate rifle attempts to make the harmonics as consistent as possible. This is generally done by "free floating" the barrel. This is done by opening up the stock channel under the barrel so that there is a small amount of space free—the general rule of thumb is enough to slide a business card between stock and barrel. This will allow the barrel to ring at the same frequency even when the barrel heats and expands, or when the stock swells and contracts with changes in humidity. Some rifles, particularly the Ruger 10/22, shoot even better with a small amount of upward force on the barrel near the end of the stock. In the case of the 10/22, this is due to the weak barrel to receiver junction, which is not strong enough to allow a free floated barrel. On other rifles (often .22 Long Rifle calibers) they just shoot better that way, even though the pressure isn't needed to support the barrel.
Stiffness without weight
One method commonly used is to use a fluted barrel. A fluted barrel is made by cutting shallow longitudinal grooves, called flutes, in the outside of a heavy barrel. This process removes weight, but retains the larger diameter of the original heavy barrel. A fluted barrel is generally stiffer than a solid barrel of the same weight, but not quite as stiff as the original heavy barrel. Fluted barrels are often favored for their ability to cool faster, as the flutes increase the surface area and thus the ability of the barrel to transfer heat to the air.
A fairly recent innovation is the composite barrel, called an "ultra-light bull barrel", or in some cases a "tensioned barrel". These are made by turning a steel barrel down to the minimum diameter suitable for the internal ballistics of the cartridge—generally a very aggressive taper from the breech end, flattening out towards the muzzle. The resulting barrel is obviously too light to be at all accurate, and looks too fragile to carry its own weight, but fulfills the requirement of containing the pressure of the propellant. Over this skinny barrel is placed a cylindrical sleeve the diameter of a typical bull barrel for the rifle in question. This sleeve is generally made from either carbon fiber composite or aluminum, and may contact the steel barrel (now called a liner) along its entire length, or only at breech and muzzle. Generally, the sleeve is held on by threading the barrel and attaching a nut on the end. The nut is then tightened down, tensioning the liner and compressing the sleeve. When compressed, the sleeve becomes even stiffer than it originally was. The resulting barrel is far lighter than a solid steel barrel of the same diameter (often half the weight or less), and is comparable or even superior in stiffness and accuracy. The downside to these barrels is that they require more effort to manufacture than a standard heavy or bull barrel, and are thus generally more expensive.
Barrel and load tuning
The other approaches to dealing with harmonics take the opposite view—rather than trying to eliminate the harmonics, they are embraced and used. For these methods, it is essential that the barrel be free floating, or bedded in a soft, flexible bedding material, so that it is allowed to vibrate freely. The goal of tuning is to have the bullet exit at the extreme end of the vibration, be it top, bottom, or side. This is the point at which the barrel is moving the slowest, so the angular dispersion of the bullets due to the vibrations will be minimized. These methods work best with long, thin barrels, since these barrels vibrate at the lowest frequency and therefore have the widest "sweet spot" when tuned correctly. Since long, thin barrels tend to be the least accurate "out of the box", the improvements that can be made by tuning are highly dramatic. It is not uncommon to see group size cut in half or more with a properly tuned combination.
The first and oldest method is tuning the load to the rifle. This is a trial and error process, and can be quite time consuming. A large number of loads are chosen, either by handloading or by purchasing a wide range of commercial ammunition. Each load is fired at a target, and the total spread of the rounds measured. The group with the smallest spread is then considered the "tuned" load for that rifle. This approach has its drawbacks, especially when rimfire cartridges are involved, as they cannot be reloaded and the user is limited to commercially available choices. Even when reloading, it is possible that the load chosen may be unsuitable for the desired purpose; the bullet weight needed for best accuracy may be too light for heavy game, or too heavy to generate the flat trajectory needed for varmint shooting.
The other method involves tuning the barrel to a given load. Since it is not practical to shave off or weld metal back onto the barrel, tuning the barrel involves having adjustable parts that can change the harmonics of the barrel. The most popular of these is the adjustable barrel weight. These are mounted either on the muzzle (lightest and most effective) or near the muzzle (shorter overall length) of the barrel. They can be moved back and forth along the length of the barrel and locked rigidly in place, and the position of the extra mass changes the frequency of the barrel—further out gives a lower frequency. The Browning BOSS system is such a tuner, and mounts to the muzzle end of a specially threaded barrel. The BOSS weight has markings similar to a micrometer plus a locking nut, so it can be dialed into a given location and locked in place. The advantage of this system is that the barrel can be tuned to a given load and that tuning recorded; that allows the rifle to be tuned to any number of different loads, and with a quick turn of a dial, the tuning for a given load can be recalled.
The disadvantage to a tuning weight is that it adds either bulk or length to the barrel, which many shooters dislike. Another system for tuning works by selectively damping the barrel, and it can be completely hidden in the stock. These systems use an adjustable pressure point mounted in the stock, pushing up on the barrel. A small screw is exposed, usually on the muzzle end of the stock, that slides a plastic block forwards and backwards along the barrel. This plastic block is mounted so that there is a significant amount of pressure up on the barrel; one product suggests a pressure of about 7 to 10 pounds (30 to 45 N). Once in place and pressing upwards on the barrel, moving the pressure block back and forth over the range of an inch or two will damp vibrations in the barrel to differing degrees.
- Tack Driving Tactical Rifle from Tac Ops - Detailed overview of the accurization process for a .25 MOA rifle
- Information on cryogenic treatment of steel for accurizing
- The Custom-Built Handgun, by Larry Leutenegger, an article on how the 1911 is customized for bullseye shooting
- Accurizing the Mini-14
- Barrel tuners for the Mini-14
- Article on AccuMajic Accurizer internal barrel tuning system that uses an adjustable pressure point for damping
- Overtravel adjustment for the Ruger MK II Target model
- Gun-Tests.com article on accurizing the Beretta 92 pistol
- Performance Shooter .38 Special wadcutter test, December 1996