Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel. The study of internal ballistics is important to designers and users of firearms of all types, from small-bore Olympic rifles and pistols, to high-tech artillery.
Contents |
The first step to firing a firearm of any sort is igniting the propellant. The earliest firearms were cannons, which were simple closed tubes. There was a small aperture, the "touchhole", drilled in the closed end of the tube, leading to the main powder charge. This hole was filled with finely ground powder, which was then ignited with a hot ember or torch. With the advent of hand-held firearms, this became an undesirable way of firing a gun. Holding a burning stick while trying to pour a charge of black powder carefully down a barrel is dangerous, and trying to hold the gun with one hand while simultaneously aiming at the target and looking for the touchhole makes it very difficult to fire accurately.
The first attempt to make the process of firing a small arm easier was the "matchlock". The matchlock incorporated a "lock" (so called because of its resemblance to door locks of the day) that was actuated by a trigger, originally called a "tricker." The lock was a simple lever which pivoted when pulled, and lowered the match down to the touchhole. The match was a slow burning fuse made of plant fibers that were soaked in a solution of nitrates, charcoal, and sulfur, and dried. This "slow-match" was ignited before the gun was needed, and it would slowly burn, keeping a hot ember at the burning end. After the gun was loaded and the touchhole primed with powder, the burning tip of the match was positioned so that the lock would bring it into contact with the touchhole. To fire the gun, it was aimed and the trigger pulled. This brought the match down to the touchhole, igniting the powder. With careful attention the slow-burning match could be kept burning for long periods of time, and the use of the lock mechanism made fairly accurate fire possible.
The next revolution in ignition technology was the "wheel-lock". It used a spring-loaded, serrated steel wheel which rubbed against a piece of iron pyrite, similar to a modern lighter. A key was used to wind the wheel and put the spring under tension. Once tensioned, the wheel was held in place by a trigger. When the trigger was pulled, the serrated edge of the steel rubbed against the pyrite, generating sparks. These sparks were directed into a pan, called the "flash pan", filled with loose powder which led into the touchhole. The flashpan usually was protected by a spring-loaded cover that would slide out of the way when the trigger was pulled, exposing the powder to the sparks. The wheel-lock was a major innovation — since it did not rely on burning material as a source of heat, it could be kept ready for extended periods of time. The covered flashpan also provided some ability to withstand bad weather. Wind, rain, and wet weather would render a matchlock useless, but a wheel-lock that was loaded and waterproofed with a bit of grease around the flashpan could be fired under most conditions.
The wheel-lock enjoyed only a brief period of popularity before being superseded by a simpler, more robust design. The "flintlock", like the wheel-lock, used a flashpan and a spark to ignite the powder. As the name implies, the flintlock used flint rather than iron pyrite. The flint was held in a spring-loaded arm, called the "cock" from the resemblance of its motion to a pecking chicken. The cock rotated through approximately a 90 degree arc, and was held in the tensioned, or "cocked" position by a trigger. Usually, flintlocks could lock the cock in two positions. The "half-cock" position held the cock halfway back, and used a deep notch, so that pulling the trigger would not release the cock. Half-cock was a safety position, used when loading, storing or carrying a loaded flintlock. The "full-cock" position held the cock all the way back, and was the position from which the gun was fired. The L-shaped "frizzen" was the other half of the flintlock's ignition system. It served as both a flashpan cover and a steel striking surface for the flint. The frizzen was hinged and spring-loaded so that it would lock in the open or closed position. When closed, the striking surface was positioned so that the flint would strike at the proper angle to generate a spark. The striking flint would also open the frizzen, exposing the flashpan to the spark. The flintlock mechanism was simpler and stronger than the wheel-lock, and the flint and steel provided a good, reliable source of ignition. The flintlock remained in military service for over 200 years, and flintlocks are still made today for historical re-enactments and muzzle-loading target competition, and for hunters who enjoy the additional challenge that the flintlock provides.
The next major leap in ignition technology was the invention of the chemical primer, or "cap", and the mechanism which used it, called the "caplock". Percussion ignition was invented by Scottish clergyman Rev. Alexander John Forsyth in 1807 but needed further refinements before it was gradually accepted in the 1820s to 1830s. By the middle of the 19th century the percussion or caplock system was well established. It was adopted by both sides in the American Civil War, as it was simpler and more reliable than the flintlock. The main reason the caplock was so quickly adopted was its similarity to the flintlock and the ease of converting older arms to use percussion-cap ignition; usually the same lock and barrel could be used with minor changes. The flashpan and frizzen were removed and replaced by a small, hollow horizontal cylinder (drum) screwed into the bored-out and tapped flash hole and carrying a "nipple" over which the cap could be fitted. A "hammer" which also had half-cock (for loading and applying the cap) and full-cock positions replaced the cock. When released by pulling the trigger, the hammer would strike the cap, crushing it against the nipple. The percussion cap was a thin metal cup that contained a small quantity of pressure-sensitive explosive. When crushed, the explosive would detonate, sending a stream of hot gas down through a hole in the nipple and into the touchhole of the gun to ignite the powder charge. In the process of firing, the cap generally split open and would fall off when the hammer was moved to half-cock position for loading. The caplock system worked well, and is still the preferred method of ignition for hunters and recreational shooters who use muzzle-loading arms.
Chemical primers, advanced metallurgy and manufacturing techniques all came together in the 19th century to create an entirely new class of firearm — the cartridge arm. Flintlock and caplock shooters had long carried their ammunition in paper cartridges, which served to hold a measured charge of powder and a bullet in one convenient package; the paper also served to seal the bullet in the bore. Still, the source of ignition was handled separately from the cartridge. With the advent of chemical primers, it was not long before several systems were invented with many different ways of combining bullet, powder, and primer into a single package which could be loaded quickly from the breech of the firearm. This greatly streamlined the reloading procedure and paved the way for semi- and fully automatic firearms.
This big leap forward came at a price. It introduced an extra component into each round – the cartridge case - which had to be removed before the gun could be reloaded. While a flintlock, for example, is immediately ready to be reloaded once it has been fired, adopting brass cartridge cases brought in the problems of extraction and ejection. The mechanism of a modern gun not only must load and fire the piece, but also must remove the spent case, which may require just as many moving parts. Probably most malfunctions involve this process, either through failure to extract a case properly from the chamber or by allowing it to jam the action. Nineteenth-century inventors were reluctant to accept this added complication and experimented with a variety of self-consuming cartridges before acknowledging that the advantages of brass cases far outweighed their one drawback.
The three systems of self-contained metallic cartridge ignition which have survived the test of time are the rimfire, the Berdan centerfire primer, and the Boxer centerfire primer.
Rimfire cartridges use a thin brass case with a hollow bulge, or rim, around the back end. This rim is filled during manufacture with an impact-sensitive primer. In the wet state, the primer is stable; a pellet of wet primer is placed in the shell and simply spun out to the full extremes of the rim. (For more on the exact process and one set of chemical compounds that have been used successfully, see U.S. Patent 1,880,235, a 1932 Remington Arms patent by James E. Burns.) In the dry state, the primer within the rim becomes impact-sensitive. When the rim is then crushed by the hammer or firing pin, the primer detonates and ignites the powder charge. Rimfire cartridges are single-use and normally cannot be reloaded. Also, since the rim must be thin enough to be easily crushed, the peak pressure possible in the case is limited by the strength of this thin rim. Rimfire cartridges originally were available in calibers up to .44, the latter used in the famous Henry and 1866 Winchester lever-action repeating rifles, but all but the small .22 caliber rounds eventually died out. The .22 Long Rifle, also fired in pistols, is the most popular recreational caliber today because it is inexpensive and quiet and has very low recoil. The most inexpensive brands can be bought for less than US$0.02 per round in cartons of 500, and even the precision Olympic class ammunition is around US$0.20 per round.
While the rimfire priming method is limited due to the thin cases required, it has enjoyed a few resurgences recently. First was Winchester's .22 Magnum Rimfire, or .22 WMR, in the 1950s, followed in 1970 by Remington's short-lived 5mm Rimfire, based on Winchester's magnum case. In 2002 Hornady introduced a new .17 caliber cartridge based on the .22 WMR, the .17 HMR. The .17 HMR is essentially a .22 WMR cartridge necked down to accept a .17-caliber bullet, and is used as a flat-shooting, light-duty varmint round. The .17 HMR was followed a year later by Hornady's .17 Mach 2, or .17 HM2, which is based on a slightly lengthened and necked-down .22 Long Rifle cartridge. Both of the .17 caliber rimfires have had widespread support from firearms makers, and while the high-tech, high-velocity .17 caliber jacketed bullets make the .17 Rimfire cartridges quite a bit more expensive than the .22 caliber versions, they are excellent for shorter-range shooting and still far less expensive than comparable centerfire cartridges.
A pinfire firearm cartridge is an obsolete type of brass cartridge in which the priming compound is ignited by striking a small pin which protrudes radially from just above the base of the cartridge. Invented by Casimir Lefaucheaux in 1828 but not patented until 1835, it was one of the earliest practical designs of metallic cartridge. However, the protruding pin was vulnerable to damage, displacement and accidental ignition. Moreover, the pin had to be positioned carefully in a small notch when loading, making the pinfire's use in repeating or self-loading weapons impossible. The pinfire survives today only in a few very small blank cartridges designed as noisemakers and in novelty miniature guns.
This unique system, much like a refined combination of the pinfire and rimfire, uses a firing pin that strikes a ring of priming compound in the center of the cartridge as described in U.S. Patent 4,848,237. Despite its being successful, only experimental batches of the cartridge were made. The primary advantage is that it is struck from the side, which allows the operating system of the firearm to be moved forward allowing a more compact action. No commercial weapons used the system, however.
The identifying feature of centerfire ammunition is the primer -- a metal cup containing primary explosive inserted into a recess in the center of the base of the cartridge. The firearm firing pin crushes this explosive between the cup and an anvil to produce hot gas and a shower of incandescent particles to ignite the powder charge.[1] Berdan and Boxer cartridge primers are both considered "centerfire". Various priming mixtures have been used in different sized primers to effect prompt ignition of the powder charge. Particles with relatively high heat capacity are required to promptly ignite smokeless powder deterrent coatings. Some priming explosives decompose into incandescent solids or liquids. Inert ingredients may be heated into incandescent sparks when the explosive decomposes into gas. Cartridges for military use require stable priming formulations so war reserves of small-arms ammunition will dependably function after years of storage.[2]
A very small but growing number of civilian and military arms are switching to electrical triggers. These use an electrical charge, powered by a battery, to detonate the primer and decrease the time between pulling the trigger and ignition of the charge. The control circuitry attendant with electrical triggers also offers opportunities for biometric safety locks, remote trigger mountings, and remote or computer-controlled operation of the weapon. Modern Gatling-type miniguns and aircraft cannon use electrical-primed ammunition due to the high rates of fire they achieve. The mechanical system of firing the primers cannot operate reliably at these extreme speeds, which reach 1,500 to 6,000 rounds per minute. These weapons have electric motors that rotate multiple barrels. As each barrel comes to the firing position, the primer passes an electrode that initiates the explosive train to the propellant, firing the cartridge.
Gunpowder (Black powder) is a finely ground, pressed and granulated mechanical mixture of sulfur, charcoal, and potassium nitrate or sodium nitrate. It can be produced in a range of grain sizes. The size and shape of the grains can increase or decrease the relative surface area, and change the burning rate significantly. The burning rate of black powder is relatively insensitive to pressure, meaning it will burn quickly even without confinement,[3] making it also suitable for use as a low explosive. However, it is a very poor explosive compared to modern high explosives because it has a very slow decomposition rate, and therefore a very low brisance. It is not, in the strictest sense of the term, an explosive, but a "deflagrant", as it does not detonate but decomposes by deflagration due to its subsonic mechanism of flame-front propagation.
Nitrocellulose or "guncotton" is formed by the action of nitric acid on cellulose fibers. It is a highly combustible fibrous material that deflagrates rapidly when heat is applied. It also burns very cleanly, burning almost entirely to gaseous components at high temperatures with little smoke or solid residue. The burning rate of nitrocellulose is dependent upon the pressure — a pile of uncontained nitrocellulose will burn slowly, with a high, bright flame, but when placed in a high-strength, sealed container, the same material will burn very quickly, bursting the container.
Gelatinised nitrocellulose is a plastic, which can be formed into many shapes of gun propellants such as cylinders, tubes, balls, and flakes. The size and shape of the propellant grains can increase or decrease the relative surface area, and change the burn rate significantly. Additives and coatings can be added to the propellant to further modify the burn rate. Normally, very fast powders are used for light-bullet or low-velocity pistols and shotguns, medium-rate powders for magnum pistols and light rifle rounds, and slow powders for large-bore heavy rifle rounds.[4] These are known as Single-base propellants.
To further increase the energy of smokeless powder, nitroglycerin can be added in amounts up to 50%. These powders are called "double-base powders", since both their main components actively produce energy, and they have similar basic physical properties to single-base powders. The nitrocellulose serves to desensitize the highly unstable nitroglycerin, preventing it from detonating as a high explosive, and the nitroglycerin gelatinises the nitrocellulose and greatly increases the energy density of the resulting powder. Double-base powders burn faster than single-base powders of the same shape, though not as cleanly, and in general the higher the nitroglycerin content of a powder, the faster the burn rate.
In artillery, Ballistite or Cordite has been used in the form of rods, tubes, slotted-tube, perforated-cylinder or multi-tubular; the geometry being chosen to provide the required burning characteristics. (Round balls or rods, for example, are "degressive-burning" because their production of gas decreases with their surface area as the balls or rods burn smaller; thin flakes are "neutral-burning," since they burn on their flat surfaces until the flake is completely consumed. The longitudally perforated or multi-perforated cylinders used in large, long-barreled rifles or cannon are "progressive-burning;" the burning surface increases as the inside diameter of the holes enlarges, giving sustained burning and a long, continuous push on the projectile to produce higher velocity without increasing the peak pressure unduly. Progressive-burning powder compensates somewhat for the pressure drop as the projectile accelerates down the bore and increases the volume behind it.)
A recent topic of research has been in the realm of "caseless ammunition". In a caseless cartridge, the propellant is cast as a single solid grain, with the priming compound placed in a hollow at the base, and the bullet attached to the front. Since the single propellant grain is so large (most smokeless powders have grain sizes around 1 mm, but a caseless grain will be perhaps 7 mm diameter and 15 mm long), the relative burn rate must be much higher. To reach this rate of burning, caseless propellants often use moderated explosives, such as RDX. (Caseless ammunition might be considered a return to the mid-19th century, since the first practical cartridge repeater, the "Volcanic" pistol, used a charge of black powder in a cavity in the bullet base. This weapon was the direct ancestor of the Henry and Winchester rifles, though they switched to metal-cased ammunition. Some early rifles and revolvers also used combustible-paper cartridges, but they required a separate ignition system.) The major advantages of a successful caseless round would be elimination of the need to extract and eject the spent cartridge case, permitting higher rates of fire and a simpler mechanism, and also reduced ammunition weight by eliminating the weight (and cost) of the brass or steel case.
While there is at least one experimental military rifle (the H&K G11), and one commercial rifle (the Voere VEC-91), that use caseless rounds, they are meeting little success. One other commercial rifle was the Daisy VL rifle made by the Daisy Air Rifle Co. and chambered for .22 caliber caseless ammunition that was ignited by a hot blast of compressed air from the lever used to compress a strong spring like for an air rifle. The caseless ammunition is of course not reloadable, since there is no casing left after firing the bullet, and the exposed propellant makes the rounds less rugged. Also, the case in a standard cartridge serves as a seal, keeping gas from escaping the breech. Caseless arms must use a more complex self-sealing breech, which increases the design and manufacturing complexity. Another unpleasant problem, common to all rapid-firing arms but particularly problematic for those firing caseless rounds, is the problem of rounds "cooking off". This problem is caused by residual heat from the chamber heating the round in the chamber to the point where it ignites, causing an unintentional discharge.
Belt-fed machine guns or magazine-fed submachine guns designed for high volumes of fire usually fire from an open bolt, with the round not chambered until the trigger is pulled, and so there is no chance for the round to cook off before the operator is ready. Such weapons could use caseless ammunition effectively. Open-bolt designs are generally undesirable for anything but belt-fed machine guns and pistol-cartridge submachine guns; the mass of the bolt moving forward causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun. Since one of the motivating factors for the use of caseless rounds is to increase the rate of fire to the degree that several shots can be fired to the same point of aim, anything that reduces the accuracy of those first shots would be counterproductive. Cased ammunition serves as a heat sink, to carry heat away from the chamber after firing; the hot case carries away much of the heat before it can transfer to the chamber walls, and the new case absorbs heat from the chamber, reducing the risk of cook-off.
Load density is the percentage of the space in the cartridge case that is filled with powder. In general, loads close to 100% density (or even loads where seating the bullet in the case, compresses the powder) ignite and burn more consistently than lower-density loads. In cartridges surviving from the black-powder era (examples being .45 Colt, .45-70 Government), the case is much larger than is needed to hold the maximum charge of high-density smokeless powder. This extra room allows the powder to shift in the case, piling up near the front or back of the case and potentially causing significant variations in burning rate, as powder near the rear of the case will ignite rapidly but powder near the front of the case will ignite later. This change has less impact with fast powders. Such high-capacity, low-density cartridges generally deliver best accuracy with the fastest appropriate powder, although this keeps the total energy low due to the sharp high-pressure peak.
Magnum pistol cartridges reverse this power/accuracy tradeoff by using lower-density, slower-burning powders that give high load density and a broad pressure curve. The downside is the increased recoil and muzzle blast from the high powder mass, and high muzzle pressure. The advantage is that the magnum pistol rounds can generate accuracy comparable to a good hunting rifle, and energy sufficient to take medium game at ranges out to 100 yards (100 m) and beyond.
Most rifle cartridges have a high load density with the appropriate powders. Rifle cartridges tend to be bottlenecked, with a wide base narrowing down to a smaller diameter, to hold a light, high-velocity bullet. These cases are designed to hold a large charge of low-density powder, for an even broader pressure curve than a magnum pistol cartridge. These cases require the use of a long rifle barrel to extract their full efficiency, although they are also chambered in rifle-like pistols (single-shot or bolt-action) with barrels of 10 to 15 inches (25 to 38 cm).
One unusual phenomenon occurs when dense, low-volume powders are used in large-capacity rifle cases. Small charges of powder, unless held tightly near the rear of the case by wadding, can apparently detonate when ignited, sometimes causing catastrophic failure of the firearm. The mechanism of this phenomenon is not well known, and generally it is not encountered except when loading low recoil or low-velocity subsonic rounds for rifles. These rounds generally have velocities of under 1100 ft/s (320 m/s), and are used for indoor shooting, in conjunction with a suppressor or for pest control, where the power and muzzle blast of a full-power round is not needed or desired.
Straight walled cases were the standard from the beginnings of cartridge arms. With the low burning speed of black powder, the best efficiency was achieved with large, heavy bullets, so the bullet was the largest practical diameter. The large diameter allowed a short, stable bullet with high weight, and the maximum practical bore volume to extract the most energy possible in a given length barrel. There were a few cartridges that had long, shallow tapers, but these were generally an attempt to use an existing cartridge to fire a smaller bullet with a higher velocity and lower recoil. With the advent of smokeless powders, it was possible to generate far higher velocities by using a slow smokeless powder in a large volume case, pushing a small, light bullet. The odd, highly tapered 8 mm Lebel, made by necking down an older 11 mm black powder cartridge, was introduced in 1886, and it was soon followed by the 7.92 x 57 mm Mauser and 7 x 57 mm Mauser military rounds, and the commercial .30-30 Winchester, all of which were new designs built to use smokeless powder. All of these have a distinct shoulder that closely resembles modern cartridges, and with the exception of the Lebel they are still chambered in modern firearms even though the cartridges are over a century old.
When selecting a rifle cartridge for maximum accuracy, a short, fat cartridge with very little case taper will generally yield higher efficiency and more consistent velocity than a long, thin cartridge with a lot of case taper (part of the reason for a bottle-necked design).[5] Given current trends towards shorter and fatter cases, such as the new Winchester Super Short Magnum cartridges, it appears the ideal might be a case approaching spherical inside.[6] Target and varmint hunting rounds require the greatest accuracy, so their cases tend to be short, fat, and nearly untapered with sharp shoulders on the case. Short, fat cases also allow short-action weapons to be made lighter and stronger for the same level of performance. The trade-off for this performance is fat rounds which take up more space in a magazine, sharp shoulders that do not feed as easily out of a magazine, and less reliable extraction of the spent round. For these reasons, when reliable feeding is more important than accuracy, such as with military rifles, longer cases with shallower shoulder angles are favored. There has been a long-term trend however, even among military weapons, towards shorter, fatter cases. The current 7.62 x 51 mm NATO case replacing the longer .30-06 Springfield is a good example, as is the new 6.5 Grendel cartridge designed to increase the performance of the AR-15 family of rifles and carbines.
Since the burning rate of smokeless powder varies directly with the pressure, the initial pressure buildup has a significant effect on the final velocity, especially in cartridges with fast powders. The friction, holding the bullet in the case, determines how soon after ignition the bullet moves, and since the motion of the bullet increases the volume and drops the pressure, a difference in friction can change the slope of the pressure curve. In general, a tight fit is desired, to the extent of crimping the bullet into the case. In straight-walled rimless cases, such as the .45 ACP, an aggressive crimp is not possible, since the case is held in the chamber by the mouth of the case, but sizing the case to allow a tight interference fit with the bullet, can give the desired result.
The bullet must tightly fit the bore to seal the high pressure of the burning gun powder. This tight fit generates a large quantity of friction. The friction of the bullet in the bore does have a slight impact on the final velocity, but that is generally not much of a concern. Of greater concern is the heat that is generated due to the friction. At velocities of about 1,000 ft/s (300 m/s), lead begins to melt, and deposit in the bore. This lead build-up constricts the bore, increasing the pressure and decreasing the accuracy of subsequent rounds, and is difficult to scrub out without damaging the bore. Rounds, used at velocities up to 1,500 ft/s (460 m/s), can use wax lubricants on the bullet to reduce lead build-up. At velocities over 1,500 ft/s (460 m/s), nearly all bullets are jacketed in copper, or a similar alloy that is soft enough not to wear on the barrel, but melts at a high enough temperature to reduce build-up in the bore. Copper build-up does begin to occur in rounds that exceed 2,500 ft/s (760 m/s), and a common solution is to impregnate the surface of the bullet with molybdenum disulfide lubricant. This reduces copper build-up in the bore, and results in better long-term accuracy.
In the first few inches (centimeters) of travel down the bore, the bullet reaches a significant percentage of its final velocity, even for high-capacity rifles, with slow burning powder. The acceleration is on the order of tens of thousands of gravities, so even a projectile as light as 40 grains (2.6 g), can provide hundreds of pounds-force (over 1000 newtons) of resistance, due to inertia. Changes in bullet mass, therefore, have a huge impact on the pressure curves of smokeless powder cartridges, unlike black powder cartridges. The loading or reloading of smokeless cartridges thus requires high-precision equipment, and carefully measured tables of load data for given cartridges, powders, and bullet weights.
Energy is imparted to the bullet in a firearm by the pressure of the gases produced by the burning gunpowder. While it seems to casual observers that a higher peak pressures should produce higher velocities, that is not always the case, since measures of peak pressure capture only a small fraction of the time the bullet is accelerating. To achieve maximum performance, the entire duration of the bullet's travel through the barrel must be considered.
There are hundreds of powders in existence because powders must be carefully matched to the case volume, case dimensions, bullet dimensions, bullet weight, barrel length, and special bullet features such as moly coating or driving bands. For example, long, heavy bullets are required to be seated so deep in the case that they displace powder, while at the same time requiring a slower powder which gives their greater mass more time to move down the barrel. If the bullet is banded or coated with a lubricant like moly, faster powders can be used as the bullet moves faster due to decreased friction with the barrel. All of these variables must be accommodated within the maximum pressure levels set for the platform. Finding the optimum combination is largely a trial and error process, and may take years to complete. New cartridges with significantly new internal ballistics often bring forth new powders engineered to maximize performance; examples of this are Accurate Arms 2230, designed for use in the .223 Remington, and #9, designed for use in magnum pistol cartridges.[7][8]
Propellant burns by consuming the outer surface of each grain of the charge. Thus, the larger the surface of the propellant's grains exposed to burn - the faster the release of gasses to the chamber and the higher the pressure buildup. This pattern of buring by external surfaces is known as Piobert's Law.
Using powder that is too fast creates a destructive pressure spike that usually has a very short duration. Using powder that is too slow produces poor energy and leaves a lot of unburned powder.
Energy is defined as a force exerted over a distance; for example, the work required to lift a one-pound weight, one foot against the pull of gravity defines a foot-pound of energy (One joule is equal to the energy used to move a body over a distance of one meter using one newton of force). If we were to modify the graph to reflect force (the pressure exerted on the bullet multiplied by its area) as a function of distance, the area under that curve would be the total energy imparted to the bullet. From this, it can be seen that the way to increase the energy of the bullet is to increase the area under that curve, either by raising the average pressure, or increasing the distance, the bullet travels under pressure (in other words, lengthen the barrel).
Another issue to consider, when choosing a powder burn rate, is the time the powder takes to completely burn vs. the time the bullet spends in the barrel. Since the burn rate of nitrocellulose-based powders increases with increasing pressure, this can be a very difficult interaction to guess, and requires careful testing with gradual changes. Looking carefully at the left graph, there is a change in the curve, at about 0.8 ms. This is the point at which the powder is completely burned, and no new gas is created. With a faster powder, burnout occurs earlier, and with the slower powder, it occurs later. Propellant that is unburned when the bullet reaches the muzzle is wasted — it adds no energy to the bullet, but it does add to the recoil and muzzle blast. For maximum power, the powder should burn until the bullet is just short of the muzzle.
Since smokeless powders burn, not detonate, the reaction can only take place on the surface of the powder. Smokeless powders come in a variety of shapes, which serve to determine how fast they burn, and also how the burn rate changes as the powder burns. The simplest shape is a ball powder, which is in the form of round or slightly flattened spheres. Ball powder has a comparatively small surface-area-to-volume ratio, so it burns comparatively slowly, and as it burns, its surface area decreases. This means as the powder burns, the burn rate slows down.
To some degree, this can be offset by the use of a retardant coating on the surface of the powder, which slows the initial burn rate and flattens out the rate of change. Ball powders are generally formulated as slow pistol powders, or fast rifle powders.
Flake powders are in the form of flat, round flakes which have a relatively high surface-area-to-volume ratio. Flake powders have a nearly constant rate of burn, and are usually formulated as fast pistol or shotgun powders. The last common shape is an extruded powder, which is in the form of a cylinder, sometimes hollow. Extruded powders generally have a lower ratio of nitroglycerin to nitrocellulose, and are often progressive burning — that is, they burn at a faster rate as they burn. Extruded powders are generally medium to slow rifle powders.
From the pressure graphs, it can be seen that the residual pressure in the barrel as the bullet exits is quite high, in this case over 16 kpsi / 110000 kPa. / 1100 bar. While lengthening the barrel or reducing the amount of propellant gas will reduce this pressure, that often is not possible due to issues of firearm size and minimum required energy. Short-range target guns usually are chambered for .22 Long Rifle or .22 Short, which have very tiny powder capacities and little residual pressure. When higher energies are required for long-range shooting, hunting or anti-personnel use, high muzzle pressures are a necessary evil. With these high muzzle pressures come increased flash and noise from the muzzle blast, and, due to the large powder charges used, higher recoil. Recoil includes the reaction caused not just by the bullet, but also by the powder mass (the residual gases acting as a rocket exhaust.)
A firearm, in many ways, is like a piston engine on the power stroke. There is a certain amount of high-pressure gas available, and energy is extracted from it by making the gas move a piston — in this case, the projectile is the piston. The swept volume of the piston determines how much energy can be extracted from the given gas. The more volume that is swept by the piston, the lower is the exhaust pressure (in this case, the muzzle pressure). Any remaining pressure at the muzzle or at the end of the engine's power stroke represents lost energy.
To extract the maximum amount of energy, then, the swept volume is maximized. This can be done in one of two ways — increasing the length of the barrel or increasing the diameter of the projectile. Increasing the barrel length will increase the swept volume linearly, while increasing the diameter will increase the swept volume as the square of the diameter. Since barrel length is limited by practical concerns to about arm's length for a rifle and much shorter for a handgun, increasing bore diameter is the normal way to increase the efficiency of a cartridge. The limit to bore diameter is generally the sectional density of the projectile (see external ballistics). Larger-diameter bullets of the same weight have much more drag, and so they lose energy more quickly after exiting the barrel. In general, most handguns use bullets between .355 (9 mm) and .45 (11.5 mm) caliber, while most rifles generally range from .223 (5.56 mm) to .32 (8 mm) caliber. There are many exceptions, of course, but bullets in the given ranges provide the best general-purpose performance. Handguns use the larger-diameter bullets for greater efficiency in short barrels, and tolerate the long-range velocity loss since handguns are seldom used for long-range shooting. Handguns designed for long-range shooting are generally closer to shortened rifles than to other handguns.
Another issue, when choosing or developing a cartridge, is the issue of recoil. The recoil is not just the reaction from the projectile being launched, but also from the powder gas, which will exit the barrel with a velocity even higher than that of the bullet. For handgun cartridges, with large bullets and small powder charges (a 9x19 mm, for example, might use 5 grains (320 mg) of powder, and a 115 grain (7.5 g) bullet), this is not a significant force; for a rifle cartridge (a .22-250 Remington, using 40 grains (2.6 g) of powder and a 40 grain (2.6 g) bullet), the powder charge can make for the majority of the recoil force.
There is a solution to the recoil issue, though it is not without cost. A muzzle brake or recoil compensator is a device which redirects the powder gas at the muzzle, usually up and back. This acts like a rocket, pushing the muzzle down and forward. The forward push helps negate the feel of the projectile recoil by pulling the firearm forwards. The downward push, on the other hand, helps counteract the rotation imparted by the fact that most firearms have the barrel mounted above the center of gravity. Overt combat guns, large-bore high-powered rifles, long-range handguns chambered for rifle ammunition, and action-shooting handguns designed for accurate rapid fire, all benefit from muzzle brakes.
The high-powered firearms use the muzzle brake mainly for recoil reduction, which reduces the battering of the shooter by the severe recoil. The action-shooting handguns redirect all the energy up to counteract the rotation of the recoil, and make following shots faster by leaving the gun on target. The disadvantage of the muzzle brake is a longer, heavier barrel, and a large increase in sound levels and flash behind the muzzle of the rifle. Shooting firearms without muzzle brakes and without hearing protection can eventually damage the operator's hearing; however, shooting rifles with muzzle brakes - with or without hearing protection - causes permanent ear damage.[9] (See muzzle brake for more on the disadvantages of muzzle brakes.)
Powder-to-projectile-weight ratio also touches on the subject of efficiency. In the case of the .22-250 Remington, more energy goes into propelling the powder gas than goes into propelling the bullet. The .22-250 pays for this by requiring a large case, with lots of powder, all for a fairly small gain in velocity and energy over other .22 caliber cartridges.
Nearly all small bore firearms, with the exception of shotguns, have rifled barrels. The rifling imparts a spin on the bullet, which keeps it from tumbling in flight. The rifling is usually in the form of sharp edged grooves cut as helices along the axis of the bore, anywhere from 2 to 16 in number. The areas between the grooves are known as lands.
Another system, polygonal rifling, gives the bore a polygonal cross section. Polygonal rifling is not very common, used by only a few European manufacturers as well as the American gun manufacturer Kahr Arms. The companies that use polygonal rifling claim greater accuracy, lower friction, and less lead and/or copper buildup in the barrel. Traditional land and groove rifling is used in most competition firearms, however, so the advantages of polygonal rifling are unproven.
There are three common ways of rifling a barrel, and one emerging technology:
The purpose of the barrel is to provide a consistent seal, allowing the bullet to accelerate to a consistent velocity. It must also impart the right spin, and release the bullet consistently, perfectly concentric to the bore. The residual pressure in the bore must be released symmetrically, so that no side of the bullet receives any more or less push than the rest. The muzzle of the barrel is the most critical part, since that is the part that controls the release of the bullet. Some rimfires and airguns actually have a slight constriction, called a choke, in the barrel at the muzzle. This guarantees that the bullet is held securely just before release.
To keep a good seal, the bore must be a very precise, constant diameter, or have a slight decrease in diameter from breech to muzzle. Any increase in bore diameter will allow the bullet to shift. This can cause gas to leak past the bullet, affecting the velocity, or cause the bullet to tip, so that it is no longer perfectly coaxial with the bore. High quality barrels are lapped to remove any constrictions in the bore which will cause a change in diameter.
A lapping process known as "fire lapping" uses a lead "slug" that is slightly larger than the bore and covered in fine abrasive compound to cut out the constrictions. The slug is passed from breech to muzzle, so that as it encounters constrictions, it cuts them away, and does no cutting on areas that are larger than the constriction. Many passes are made, and as the bore becomes more uniform, finer grades of abrasive compound are used. The final result is a barrel that is mirror-smooth, and with a consistent or slightly tapering bore. The hand-lapping technique uses a wooden or soft metal rod to pull or push the slug through the bore, while the newer fire-lapping technique uses specially loaded, low-power cartridges to push abrasive-covered soft-lead bullets down the barrel.
Another issue that has an effect on the barrel's hold on the bullet is the rifling. When the bullet is fired, it is forced into the rifling, which cuts or "engraves" the surface of the bullet. If the rifling is a constant twist, then the rifling rides in the grooves engraved in the bullet, and everything is secure and sealed. If the rifling has a decreasing twist, then the changing angle of the rifling in the engraved grooves of the bullet causes the rifling to become narrower than the grooves. This allows gas to blow by, and loosens the hold of the bullet on the barrel. An increasing twist, however, will make the rifling become wider than the grooves in the bullet, maintaining the seal. When a rifled-barrel blank is selected for a gun, careful measurement of the inevitable variations in manufacture can determine if the rifling twist varies, and put the higher-twist end at the muzzle.
The muzzle of the barrel is the last thing to touch the bullet before it goes into ballistic flight, and as such has the greatest potential to disrupt the bullet's flight. The muzzle must allow the gas to escape the barrel symmetrically; any asymmetry will cause an uneven pressure on the base of the bullet, which will disrupt its flight. The muzzle end of the barrel is called the "crown", and it is usually either beveled or recessed to protect it from bumps or scratches that might affect accuracy. A sign of a good crown will be a symmetric, star-shaped pattern on the muzzle end of the barrel, formed by soot deposited, as the powder gases escape the barrel. If the star is uneven, then it is a sign of an uneven crown, and an inaccurate barrel.
Before the barrel can release the bullet in a consistent manner, it must grip the bullet in a consistent manner. The part of the barrel between where the bullet exits the cartridge, and engages the rifling, is called the "throat", and the length of the throat is the freebore. In some firearms, the freebore is all but nonexistent — the act of chambering the cartridge forces the bullet into the rifling. This is common in low-powered rimfire target rifles. The placement of the bullet in the rifling ensures that the transition between cartridge and rifling is quick and stable. The downside is that the cartridge is firmly held in place, and attempting to extract the unfired round can be difficult, to the point of even pulling the bullet from the cartridge in extreme cases.
With high-powered cartridges, there is an additional disadvantage to a short freebore. A significant amount of force is required to engrave the bullet, and this additional resistance can raise the pressure in the chamber by quite a bit. To mitigate this effect, higher-powered rifles tend to have more freebore, so that the bullet is allowed to gain some momentum, and the chamber pressure is allowed to drop slightly, before the bullet engages the rifling. The downside is that the bullet hits the rifling when already moving, and any slight misalignment can cause the bullet to tip as it engages the rifling. This will, in turn, mean that the bullet does not exit the barrel coaxially. The amount of freebore is a function of both the barrel and the cartridge. The manufacturer or gunsmith who cuts the chamber will determine the amount of space between the cartridge case mouth and the rifling. Setting the bullet further forward or back in the cartridge can decrease or increase the amount of freebore, but only within a small range. Careful testing by the ammunition loader can optimize the amount of freebore to maximize accuracy, while keeping the peak pressure within limits.
The defining characteristic of a revolver is 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.[41]
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.[41][42][43]
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.[43]