Terminal ballistics is a sub-field of ballistics concerned with the behavior and effects of a projectile when it hits and transfers its energy to a target. This field is usually cited in forensic ballistics.
Bullet design (as well as the velocity of impact) largely determines the effectiveness of penetration.Terminal Ballistics Test and Analysis Guidelines for the Penetration Mechanics Branch. BRL.
General The concept of terminal ballistics can be applied to any projectile striking a target. Much of the topic specifically regards the effects of small arms fire striking live targets, and a projectile's ability to incapacitate or eliminate a target.
Common factors include bullet mass, composition, velocity, and shape.
Firearm projectiles
Class of projectile Projectiles are primarily designed for compatibility with the constraints of the device used to launch them, and secondarily according to some balance of logistical practicality, practicable accuracy, and terminal effect. Prior to the development of rifling, the majority of projectiles purpose-built for shooting consisted of fitted round balls, or multi-projectile shot loads. In modern times, this approach to shooting persists, as well as other means which have been refined within unique settings over centuries. In addition to advances of barrel design, the means of propulsion available have also diversified, including guns designed to use black powder, smokeless powder, compressed air, and electromagnetic force.
Ammunition, and the components thereof, can be categorized in an assortment of fashions. Among other ways, it may be approached according to the shape, the mass, the dimensions of a projectile or cartridge, the customary charge of powder, velocity, intended purpose, and recommended applications. Though some projectiles and ammunition are designed from the beginning for the sole purpose of target shooting, the minimal powder charges necessary to force a metal projectile down the length of a barrel may be potentially lethal, and should be treated as such.
Conventional forms of projectiles:
Round ball - Spherical in shape prior to loading, used in smooth or rifled barrels Shot - Multi-projectile load, typically spherical or semi-spherical, best used in smoothbores Slug - A nose heavy design that uses air drag from a light hollowed base or wad to aid stability Pellet - (Specific) A self sealing, non-spherical projectile, typically designed to obturate with a rear skirt Bullet - (Specific) A semi-cylindrical projectile, often with a frontal nose, stabilized gyroscopically from rotation
Target shooting
For short-range target shooting, typically on ranges up to 50 meters, or 55 yards, with low-powered ammunition like .22 long rifle, aerodynamics is relatively unimportant, and velocities are low compared to velocities attained by full-powered ammunition.
As long as a bullet's mass is balanced, it will not tumble; its shape is thus unimportant for purposes of its aerodynamics. For shooting at paper targets, bullets that will punch a perfect hole through the target —called wadcutters— are preferred. They have a very flat front, often with a relatively sharp edge along the perimeter, which punches out a hole equal to or almost equal to its diameter, thus enabling unambiguous scoring of the target. Since cutting the edge of a target ring will result in a higher score, accuracy to within fractions of an inch is desirable.
Magazine-fed pistols tend not to reliably feed wadcutters because of their angular shape. To address this, the semi-wadcutter is often used. The semi-wadcutter consists of a conical section that comes to a smaller flat point and a thin sharp shoulder at the base of the cone. The flat point punches a hole, and the shoulder opens it up cleanly. For steel targets, the concern is to provide enough force to knock over the target while minimizing the damage to the target. A soft lead bullet, or jacketed hollow-point bullet, or soft-point bullet will flatten out on impact (if the velocity at impact is sufficient to make it deform), spreading the impact over a larger area of the target, allowing more total force to be applied without damaging the steel target.
There are also specialized bullets designed for use in long-range precision target shooting with high-powered rifles. The designs vary somewhat from manufacturer to manufacturer. Research in the 1950s by the U.S. Air Force discovered that bullets are more stable in flight for longer distances and more resistant to crosswinds if the center of gravity is biased to the rear of the center of pressure. The MatchKing bullet is an open-tip match design with a tiny aperture in the jacket at the point of the bullet and a hollow air space under the point of the bullet, whereas previous conventional bullets had a lead core that went all the way up to the point.
The U.S. military now issues ammunition to snipers that use bullets of this type. M852 Match and M118LR ammunition are issued for the 7.62×51mm chamber: both use Sierra MatchKing bullets; for 5.56×45mm those U.S. Navy and U.S. Marine snipers who use accurized M16-type rifles are issued the Mk 262 Mod 0 cartridge developed jointly by Black Hills Ammunition and Crane Naval Surface Warfare Center.
For ultra-long-range precision target shooting with high-powered rifles and military sniping, radically designed very-low-drag (VLD) bullets are available that are generally produced out of rods of mono-metal alloys on CNC lathes. The driving force behind these projectiles is the wish to enhance the practical maximum effective range beyond normal standards. To achieve this, the bullets have to be very long and normal cartridge overall lengths often have to be exceeded. Common rifling twist rates also often have to be tightened to stabilize very long projectiles. Such commercially nonexistent cartridges are termed "wildcats". The use of a wildcat-based (ultra) long-range cartridge demands the use of a custom or customized rifle with an appropriately cut chamber and a fast-twist bore.
Maximum penetration For use against armored targets, or large, tough game animals, penetration is the most important consideration. Focusing the largest amount of kinetic energy and projectile mass on the smallest possible area of the target provides the greatest penetration. Bullets for maximum penetration are designed to resist deformation on impact and usually are made of lead that is covered in a copper, brass, or mild steel jacket (some are even solid copper or bronze alloy). The jacket completely covers the front of the bullet, although often the rear is left with exposed lead (this is a 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 from a copper-jacketed steel core; the steel resists deformation better than the usual soft lead core leading to greater penetration. The current NATO 5.56mm SS109 (M855) bullet uses a steel-tipped lead core to improve penetration, the steel tip providing resistance to deformation for armor piercing, and the heavier lead core (25% heavier than the previous bullet, the M193) providing increased sectional density for better penetration in soft targets. For larger, higher-velocity calibers, such as tank guns, hardness is of secondary importance to density, and are normally sub-caliber projectiles made from tungsten carbide, tungsten hard alloy, or depleted uranium fired in a light aluminum or magnesium alloy (or carbon fiber in some cases) sabot.
Many modern tank guns are smoothbore, not rifled because practical rifling twists can only stabilize projectiles, such as an armor-piercing, capped, ballistic cap, with a length-to-diameter ratio of up to about 5:1 and also because the rifling adds friction, reducing the velocity and thus total force it is possible to achieve. To get the maximum force on the smallest area, modern 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 instead of rifling. These subcaliber rounds, called armor-piercing fin-stabilized discarding sabot are held in place in the bore by sabots. The sabot is a light material that transfers the pressure of the charge to the penetrator, then is discarded when the round leaves the barrel.
Controlled penetration The final category of bullets is that intended to control penetration so as not to harm anything behind the target. Such bullets are used primarily for hunting and civilian antipersonnel use; they are not generally used by the military, since the use of expanding bullets in international conflicts is prohibited by the Hague Convention and because these bullets have less chance of penetrating modern body armor. These bullets are designed to increase their surface area on impact, thus creating greater drag and limiting the travel through the target. A desirable side effect is that the expanded bullet makes a larger hole, increasing tissue damage and speeding up incapacitation.
While a bullet that penetrates through-and-through tends to cause more profuse bleeding, allowing a game animal to be blood trailed more easily, in some applications, preventing exit from the rear of the target is more desirable. A perforating bullet can continue on (likely not coaxial to the original trajectory due to target deflection) and might cause unintended damage or injury.
Flat point One of the simpler ways to find consistent disruption from a bullet is forming a wide and flat tip. This increases the effective surface area, as rounded bullets can allow tissues to "flow" around the edges. Flat points also increase drag during flight to various extent, which along with the type of material and muzzle velocity, tends to affect the degree of expansion at impact.
Flat-point bullets, with especially pronounced fronts of up to 90% of the overall bullet diameter, are sometimes preferred for use against large or dangerous game animals. For such purposes, they are typically made of unusually hard alloys, and may be longer and heavier than normal for their caliber to decrease the chance of deflection, and even include exotic materials such as tungsten to increase their sectional density. These bullets are designed to penetrate with sufficient depth through muscle, bone, and vital areas while causing a wound channel ranging from bullet diameter to the size of a coin, significantly larger than the bullet, and are most likely to perform similarly at any angle and at various ranges. One of the hunting applications of the flat point bullet is large game such as bear hunting, in which case people may be carrying a sidearm such as a .44 Magnum, 10 mm, or a larger caliber that is not intensively reliant on expansion.
Light weight non-expanding projectiles pushed at a relatively high speed are also utilized, generally for close ranged applications well inside 100 yards. The light for caliber bullets will transfer energy to a given medium more rapidly, yet with consistent penetration characteristics relative to their sectional density and a constant tapering effect as the bullet graduates to a stop. Such implementations may be conducive to mitigating fragmentation where weight retention is prioritized.