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Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a kinetic energy projectile-when it impacts with its target. Terminal ballistics is relevant for both small calibre projectiles as for large calibre projectiles (fired from artillery). The study of extremely high velocity impacts is still very new and is as yet mostly applied to spacecraft design.
There are three basic classes of bullet: ones that are designed for maximum penetration of the target, ones that are designed to penetrate a specific depth and stop, and ones that are designed specifically for short range target shooting.
For short range target shooting on ranges up to 50 yards (50 m) aerodynamics is relatively unimportant and velocities are low. As long as the bullet is balanced so it doesn't tumble, the accuracy will be sufficient. For shooting at paper targets, the best bullet is one that will punch a perfect hole through the target. These bullets are called wadcutters, and they have a very flat front, often with a relatively sharp edge along the perimeter. The flat front punches out a large hole in the paper, close to if not equal to the full diameter of the bullet. This allows for easy, nonambiguous scoring of the target. Since cutting the edge of a target ring will result in scoring the higher score, fractions of an inch are important. In magazine fed pistols, the square shape of a wadcutter will often not feed reliably. To address this, the semiwadcutter was developed. The semiwadcutter consists of a conical section that comes to a smaller flat, and a thin sharp shoulder at the base of the cone. The flat point punches a clean hole, and the shoulder opens the hole up cleanly. For steel targets, the concern is to provide enough force to knock over the target, but to minimize the damage to the target. A soft lead bullet, or a jacketed hollow point bullet or soft point bullet will flatten out on impact, spreading the force over a larger area of the target, allowing more total force to be applied without damaging the steel target.
For use against armored targets, or large, tough game animals, penetration is the most important consideration. Focusing the largest amount of kinetic energy on the smallest possible area of the target provides the greatest penetration. Bullets for maximum penetration are designed to resist deformation upon impact, and usually are made of lead that is covered in a copper, brass, or mild steel jacket. The jacket completely covers the front of the bullet, although often the rear is left with exposed lead (this is manufacturing consideration, the jacket is formed first, and the lead is swaged in from the rear). For penetrating substances significantly harder than jacketed lead, the lead core is supplemented with or replaced with a harder material, such as hardened steel. Military armor piercing small arms ammunition is made with a steel core, and the current NATO 5.56 mm SS109 bullet contains a steel tip in front of a lead core, for enhanced penetration of body armor. For larger, higher velocity calibers, such as tank guns, hardness is of secondary importance to density, and are often sub-caliber projectiles. Oddly, many modern tank guns are smoothbore, not rifled. This is because practical rifling twists can only stabilize projectiles with a length to diameter ratio of up to about 5:1. To get the maximum force on the smallest area, anti-tank rounds have aspect ratios of 10:1 or more. Since these cannot be stabilized by rifling, they are built instead like large darts, with fins providing the stabilizing force, negating the need for rifling. These subcaliber rounds are held in place in the bore by sabots.
The final category of bullets are the controlled penetration rounds. These are used primarily for hunting and civilian antipersonnel use; they are not generally used by the military, since the use of the most common controlled penetration rounds in international conflicts is prohibited by the Hague Convention. Controlled penetration bullets are designed to increase their surface area upon impact, thus transferring more of their energy to the target, and preventing exit from the rear of the target. A bullet which passes completely through the target is not desirable, because it retains some of the energy which could have been used to disable the target, and it can then continue on (likely not coaxial to the original trajectory due to target deflection) and might cause unintended damage or injury.
The simplest controlled penetration bullet is one with a wide, flat tip. The simple lack of streamlining will limit the penetration fairly significantly. Flat point bullets, with fronts of up to 90% of the overall bullet diameter, are designed for use against large or dangerous game. They are often made of unusually hard alloys, are longer and heavier than normal for their caliber, and even include exotic materials such as tungsten to increase their sectional density. These bullets are designed to penetrate deeply through muscle and even bone, while causing a wound channel of nearly the full diameter of the bullet. These bullets are designed to penetrate deeply enough to reach vital organs, but still produce enough tissue damage to quickly kill an animal even if a vital organ is not hit. One of the common hunting applications of the flat point bullet is large game such as bear hunted with a handgun in a .44 Magnum or larger caliber. The disadvantage of flat point bullets is the reduction in aerodynamic performance; the flat point induces much drag, leading to significantly reduced velocities at long range.
More effective on lighter targets are the expanding bullets, the hollow point bullet and the soft point bullet. These are designed to use the hydraulic pressure of muscle tissue to expand the bullet. This process is called mushrooming, as the ideal result is a shape that resembles a mushroom--a cylindrical base, topped with a wide surface where the tip of the bullet has peeled back to expose more area. A copper plated hollow point loaded in a .44 Magnum, for example, with an original weight of 240 grains (16 g) and a diameter of 0.43 inches (11 mm) might mushroom on impact to form a rough circle with a diameter of 0.70 inches (18 mm) and a final weight of 239 grains (15 g). This is excellent performance; almost the entire weight is retained, and the frontal surface area increased by 165%. Penetration of the hollow point would be less than half that of a similar nonexpanding bullet, and the resulting wound cavity would be much wider.
The last category of expanding bullets are the frangible bullets. These bullets are designed to break up on impact, which results in a huge increase in surface area. The most common of these bullets are made of small diameter lead balls, placed in a thin copper shell and held in place by an epoxy or similar binding agent. Upon impact, the epoxy shatters and the copper shell opens up, much like a hollow point bullet. The individual lead balls then spread out in a wide pattern, and due to their low mass to surface area ratio, stop very quickly. Similar bullets are made out of sintered metals, which turn to powder upon impact. These bullets are usually restricted to pistol cartridges, as the nonhomogenous cores tend to cause inaccuracies that, while acceptable at short pistol ranges, are not acceptable for the typical range at which rifles are used. One interesting use of the sintered metal rounds is in shotguns in hostage rescue situations; the sintered metal round is used at near-contact range to shoot the lock mechanism out of doors. The resulting metal powder will immediately disperse after knocking out the door lock, and cause little or no damage to occupants of the room. Frangible rounds are also rumored to be used by armed security agents on aircraft. The concern is not depressurization (a bullet hole will not depressurise an airliner) but overpenetration and damage to vital electrical or hydraulic lines.
Also used are bullets similar to hollow point bullets or soft point bullets whose cores and/or jackets are deliberately weakened to cause deformation or fragmentation upon impact. The Warsaw Pact 5.45 x 39 mm assault rifle round exemplifies a trend that is becoming common in the era of high velocity, small caliber military rounds. The 5.45 x 39 mm uses a steel jacketed bullet with a 2 part core, the rear being steel and the front being lead. Upon impact, the lead deforms, bending the bullet into a slight "L" shape. This causes the bullet to tumble in the tissue, thus increasing its effective frontal surface area by traveling sideways more often than not. This does not violate the Hague Convention, as it specifically mentions bullets that expand or flatten in the body. The NATO SS109 also tends to bend at the steel/lead junction, but with its weaker jacket, it fragments into two pieces.
Other bullets in use by militaries are quite backheavy, due to a long, sharp point created in an attempt to get the maximum ballistic coefficient (see external ballistics). These bullets will flip over after impact, then settle into a stable, back first orientation before stopping. The Swiss military actually redesigned their 6.5 mm assault rifle bullet to prevent this, to more fully comply with the spirit of the Hague Convention.
A note on bullet diameter. It might seem that if the whole purpose of a controlled penetration round is to expand to a larger diameter, it would make more sense to start out with the desired diameter rather than relying on the somewhat inconsistent results of expansion upon impact. While there is merit to this (there is a strong following of the .45 ACP, as compared to the 0.355 in diameter 9 x 19 mm, for just this reason) there are also significant downsides. A larger diameter bullet is going to have significantly more drag than a smaller diameter bullet of the same mass, which means long range performance will be significantly degraded. A larger diameter bullet also means more space is required to store the ammunition, which means either bulkier guns or smaller magazine capacities. The common tradeoff when comparing .45 ACP and 9 x 19 mm pistols is a 7 or 8 round capacity in the .45 ACP vs. a 13 to 15 round capacity in the 9x19mm. Especially where the military requirement of a nonexpanding round is concerned, there is fierce debate over whether it is better to have fewer, larger bullets for enhanced terminal effects, or more, smaller bullets for increased number of potential target hits.
The standard medium for testing bullets for performance on tissue is ballistic gelatin. Performance is generally graded with two factors, the maximum depth of penetration and the size of the cavity formed in the gelatin by the bullet impact. The size of the cavity represents the amount of tissue damaged, and the hemorrhaging that will result from such a wound. The penetration represents how far into the tissue the bullet will ultimately penetrate.
For a quick incapacitation, a hit to a vital organ is needed so a bullet that will penetrate to the depth required for such a hit should be chosen. When hunting groundhogs, for example, a bullet that expands quickly to form a large cavity with minimum penetration would be the best choice. When hunting deer, a bullet that expands slower or to a smaller extent, and produces a moderate cavity with moderate penetration would be the best choice. For hunting bear, a bullet that will penetrate deeply and expand little or none would be required.
For dangerous game especially, expansion is not required--the reason for this is that the hemorrhaging caused by a large wound cavity doesn't disable the game quickly enough, so the hunter must shoot for a vital organ hit or risk being killed by a wounded animal. To hit the vital organs on a large game animal requires penetrating the thick fat and muscle tissue surrounding the chest cavity, and possibly even punching through bone. This requires a hard, nondeforming bullet.
The rules of engagement for nonmilitary use of firearms usually require that a life be in immediate danger for shots to be fired. Under such circumstances, the goal is to incapacitate the target as quickly as possible, to prevent the harm from being done. In most cases, the shots are fired from a handgun, which is, compared to a rifle, very much underpowered. Humans are in roughly the same class as deer sized game, and in most places, the minimum cartridge power required to hunt deer is more than twice that of the average police sidearm. Handguns are also very inaccurate in the hands of all but the best shooters, and the average defensive shooter is not an expert, and is under a great deal of stress, which further degrades accuracy. These factors combine to require extremely effective terminal ballistics to provide swift incapacitation of the target under far less than ideal circumstances.
Since humans walk upright and present relatively unprotected vital organ targets (it is much easier to reach vital organs from front or rear than from the sides of the chest cavity), bullets for use on humans are usually designed for about 6 to 12 inches (150 to 300 mm) of penetration. This will assure significant tissue damage from a torso shot from most angles, and a rapid incapacitation. Frangible rounds, while they are sold for defensive purposes, are not well suited for the role, as they generally penetrate less than 6 inches (150 mm), and are therefore prone to failure. When they work, they work very well, but when they fail, they tend to fail badly.
Most police departments in the USA are required to carry department approved rounds, usually a high performance hollow point bullet. Oddly, the law implicitly requires the police to carry the most devastating round they practically can. If lethal force is required, then you must stop the target quickly; using anything less, in an attempt to minimize damage to the target, would be considered an implicit allowance to use potentially lethal force in a situation that did not require lethal force.
The purpose of firing a large calibre projectile is not always the same. For example, one might need to create disorganisation within enemy troops, create casualties within enemy troops, eliminate the functioning of an enemy tank, destroy an enemy bunker, etc. Different purposes of course require different projectile designs.
Many large calibre projectiles are designed to explode or to release chemical weapons or biological weapons either on impact or when close to a target; designing an appropriate fuse is a difficult task which lies outside the realm of terminal ballistics.
Other large calibre projectiles use subprojectiles, which are released when the carrying projectile explodes close to its target. This kind of projectile aims at creating maximum casualties amongst enemy troops. A side-effect is that unexploded subprojectiles can later be found and create casualties at that time. International conventions tend to forbid or restrict the use of this type of projectile.
Projectiles designed to penetrate heavy tank armor tend to be long, narrow and very hard.
The study of projectile impacts with velocities greater than several kilometers per second is an area of active research. Such impacts are not yet used in military situations, but can arise from meteoric impact. The impact of extremely small, extremely fast particles is of interest in designing spacecraft to withstand erosion due to micrometeoroids.
Accelerating projectiles up to such speeds is currently difficult; linear motors and shaped charges are currently the most common techniques for producing such speeds.
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