Accurizing

Determining accuracy is not always a straightforward task, as it depends on a large number of variables. 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. 3 or 5-shot groups are acceptable for zeroing the sights and rough accuracy estimates, but most shooters consider 10-shot groups to be the minimum for accuracy comparisons. Defining accuracy 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 wadcutter rounds in a semi-automatic pistol and two different revolvers: a Smith & Wesson Model 52, Smith & Wesson Model 686, and a Colt Python Target model, with five, six 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 . Based on average group size, the winner was the Model 686, which shot an average group of across the brands of ammunition, with a standard deviation between ammunition types of . However, the Model 52, while shooting slightly larger groups at , was far more consistent across the brands, with a standard deviation of only , and was the most consistent performer of the test. However, if the ammunition was tuned to the gun, the clear winner was the Python, which averaged just with its favored brand of ammunition. The Python was also by far the pickiest, however, turning in the largest groups at averages with its least favorite brands, for a standard deviation of . Based on this test, answering the question "Which is the most accurate?" becomes a matter of opinion. The 686 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. Testing methodology 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 weapon will 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 good results on one may not affect another. Another issue in measuring accuracy is the method by which the gun is secured for the test. 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. ==Usability==

No matter what the potential laboratory accuracy of a firearm, it does not matter if a human shooter cannot fire it accurately under a set of real-world conditions. 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. Over travel is not always considered bad by some, as the force of the trigger finger does not impact on the gun directly after sear release. Improving the trigger pull air pistol trigger mechanism, unmodified (top) and with a sear engagement adjustment (bottom). 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. 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. 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. Sights A firearm's sights help its user to align the barrel with an intended target. In some cases the only refinement in a "target" firearm over a standard model is improved sights. Adjustable sights are essential for most target shooting, as they allow compensation for variables such as target range and ammunition type. Firearms with non-adjustable or coarsely adjustable sights cannot give their holders the ability to reliably shoot on target in changing conditions. Improvements in visibility and sharpness of a target image provided by some sights can also improve users' aim and consistency. while helping the person take better advantage of the improvement. Some of these "peep sights" provide precise, repeatable adjustments for long range shooting with no need for tools. Telescopic and reflex sights offer advantages to people with less experience or poor eyesight by bringing both target and aiming point into focus, while "scopes" also magnify and brighten the image. Drawbacks such as weight, bulk, and complexity can also affect a shooter's performance. Stocks and grips finish A good stock or grip 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". 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. 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. For shotguns in particular, the placement of the shooter's face on the stock provides the rear aiming point, and the correct drop, toe alignment, 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. 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 range and ideal trigger control. However, these features are not well suited for a typical hunting or action shooting rifle, 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. 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. 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. 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. 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. Time considerations 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 will 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 Lock time is the time between the release of the sear and the ignition of the cartridge, A short lock time is particularly desired when shooting with high precision at small targets. The lock time of conventional bolt-action rifles typically lie between 2.6 and 9.0 milliseconds. 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 ignition of the cartridge and when the bullet leaves the barrel. 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. To put lock time and bullet dwell time in perspective; the lock time of most conventional bolt action rifles varies between 2.6 and 9.0 milliseconds, while after cartridge ignition most rifle bullets travel through a high powered rifle barrel bore in 1.0 to 1.5 milliseconds. Mechanical bolt-action rifle trigger systems with a sub 2.0 millisecond lock time are applied in most purpose designed high end competition rifles. ==Clearances and Tolerances==

The terms "clearance" and "tolerance" are very often confused and misused. Clearance is the distance between the surfaces of mating parts. Tolerance is the allowable variation of a dimension from its nominal (desired) value. For example, a bolt with an outside diameter (OD) of 0.697 inches which is operating in a receiver with a bolt raceway inside diameter (ID) of 0.702 has a clearance of 0.005 inches. If the bolt OD has a nominal OD of 0.698 and a tolerance of ±0.001, then the bolt OD can randomly vary from 0.697 to 0.699 along its length, which will allow the bolt clearance in the 0.702 diameter raceway to vary from 0.005 to 0.003. Further, if the raceway also has a tolerance of ±0.001 from the nominal of 0.702, then its ID can vary from 0.701 to 0.703 along its length. That combination of tolerances can allow the bolt clearance to vary from 0.002 to 0.006. The point at which the 0.002 clearance occurs would likely cause binding and malfunction in most operational environments. To ensure consistent, repeatable lockup, clearances between moving parts must be kept to the smallest value that will allow proper operation of the mechanism. This goal can be achieved by carefully hand selecting parts and precisely fitting them together, or by manufacturing new parts (bolt, receiver, barrel, etc. to precise dimensions using much tighter tolerances than production components. The best fits are generally attained by selecting slightly oversized parts (or altering stock parts to form an interference fit) and then lapping the mating surfaces to attain the desired fit (clearance). 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. Bore Ideally, the bore must be cylindrical and the rifling geometry the same down the length of the bore. Some airgun rifles have a short cone towards the muzzle to improve the speed of the bullet. 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. 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 precess in flight; at its worst, this can result in the bullets tumbling in-flight and 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. In practical terms, this is only a problem for rifles chambered for common military calibers where a variety of different loads exist. For example, the M16A1 rifle is unable to accurately fire bullets that are heavier than due to barrel twist that is too slow to stabilize heavier bullets. Precision rifles typically come with barrels that are either tailor-made for a specific ammunition load or made according to the buyer's specifications. Barrels can 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 out and thus not interfere with the passage of the bullet. A smooth, polished bore will not only hold the bullet better, but also reduce barrel fouling. An 11 degree crown has the best accuracy potential, and should be applied to a barrel to get the best gas flow at the bullet exit point of the barrel. Stress 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. For example, the hammer forging method of manufacture leaves a significant amount of stress in the barrels, which could be addressed through stress-relieving heat treatment. Wear 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/molybdenum steels used for barrels. Other materials, such as composites or laminated wood, can also provide a stronger, more dimensionally stable stock than traditional woods can. Some stocks are even being made out of aluminum or other metals, for maximum stability. 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. 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. ==Harmonics==

During firing, the chamber pressure rises from atmospheric pressure to, in a typical rifle cartridge, pressures of about within microseconds. 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. There are two ways to address harmonics; reducing the amplitude with a stiffer barrel, or working with the natural frequency to minimize dispersion. Stiffness 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 effect 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 shooting barrels, where weight is of very little consequence, have very large diameters; an outside diameter of 2 inches (5 cm) is not uncommon. 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 and improve heat dispersion while maintaining most of the stiffness. Barrel tensioning devices are a way of gaining stiffness with minimal increases in weight. They do this by placing a lightweight sleeve, often made of aluminium or a carbon fiber composite, 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. Harmonic tuning 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 ballistics of 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, handloading gives 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. Other tuners work by using an adjustable 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. ==Airgun powerplants==

The difference between an airgun and a firearm is the way in which the power to launch the projectile is provided. In a firearm the projectile propulsion is provided by an exothermic chemical reaction, and in an airgun is it provided primarily by mechanically compressed gas, typically either air or carbon dioxide (CO2), though these gases are used primarily for convenience and some airgun variants run on other gases, such as refrigerants like R-134a commonly used in airsoft guns, or hydrogen used in light-gas guns. There are three primary types of powerplant used in airguns: • Spring-piston, which uses a spring-loaded piston to compress air within an air pump at the moment of firing • Pneumatic, which uses pre-compressed air stored in a reservoir within the gun • Compressed gas, which uses a small removable gas cylinder now ubiquitously stores liquid CO2 (Powerlet) Each method has its own advantages and disadvantages, and different areas that can be addressed to ensure consistency. The most powerful systems will produce velocities near or exceeding the speed of sound with lightweight pellets; this, however, is not a good thing where accuracy is concerned. The commonly used airgun diabolo 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 preservation of velocity and kinetic energy. Pneumatic Pneumatic systems use 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. 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, is the pump airgun's major disadvantage when it comes to accuracy, as it makes it very difficult to get a consistent charge. 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 SCUBA tank, 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. Piston 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. The 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. 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 affect the harmonics of the gun. A gas spring will, if one can be fitted to a given model, provide vibrationless action, though with some loss of efficiency and even sharper recoil. 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 accuracy. CO2 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. ==References==