The latter class may limit penetration by expanding or fragmenting,
There are also specialized bullets designed specifically for use in long range precision target shooting with high-powered rifles; the designs vary somewhat from manufacturer to manufacturer, but all are based on the MatchKing bullets introduced by the Sierra Bullet Company around 1963. Based on research done in the 1950s by the US Air Force, in which it was discovered that bullets are more stable in flight for longer distances and more resistant to crosswinds if the center of gravity is somewhat to the rear of the center of pressure, the MatchKing bullet (which is still in wide use and holds many records) is a hollowpoint 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, where previous conventional bullets had had a lead core that went all the way up to the point. Other designs from other manufacturers may be anything from close copies of the MatchKing design to hollowpoint bullets with a deep, wide cavity containing a long, slender, pointed plastic or aluminum plug. In all these cases, the bullet is designed to have its center of gravity to the rear of its center of pressure. MatchKing type hollowpoint bullets, as contrasted with hollowpoint bullets intended for hunting or police use, are not designed to flatten out on impact; this makes them a relatively poor choice for hunting, as they tend to perform erratically and unpredictably upon entering an animal's body—they may tumble, or break apart, though most often they punch straight through making a narrow wound that usually does not cause death quickly (as full metal jacket ammunition normally does). The US military now issues ammunition to snipers that use bullets of this type. In 7.62 x 51 mm NATO, M852 Match and M118LR ammunition are issued, both of which use Sierra MatchKing bullets; in 5.56 x 45 mm NATO, those US Navy and US 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, using a bullet manufactured by Sierra Bullets that was cannelured according to military specifications for this project.
In 1985, the US military Adjutant General's Office issued a legal opinion holding that the Sierra MatchKing bullet, despite being a hollowpoint design, is not designed specifically to cause greater damage or suffering in a human target, and in fact normally does not create a wound readily distinguishable from wounds caused by conventional full metal jacket bullets, and is therefore in their opinion legal under the Hague Convention for use in war
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 controlled 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. More information about these very specialized (and expensive) projectiles can be found in the very-low-drag bullet article.
Also used are bullets similar to hollowpoint 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 M74 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 many dozens of pieces. NATO 7.62 mm ball manufactured by some countries, such as Germany and Sweden, are also known to fragment due to jacket construction.
Other bullets in use by militaries are quite back heavy, 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 5.56 mm assault rifle bullet to prevent this, to more fully comply with the spirit of the Hague Convention, though according to some sources the present GP90 5.56x45mm Swiss assault rifle ammunition was actually designed as an armor-piercing bullet, because in the 1980s it was perceived that the Soviets and their Warsaw Pact allies were going to issue soft body armor to infantry units on a wide basis, but after the end of the Cold War, the Bofors corporation, having spent a great deal of money on developing the new bullet, changed the sales pitch in order to sell it to the Swiss government.
It might seem that if the whole purpose of a maximum disruption 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 trade-off when comparing .45 ACP and 9 x 19 mm pistols is a 7 to 12 round capacity in the .45 ACP vs. a 13 to 18 round capacity in the 9x19 mm. Although several .45-caliber pistols are available with high-capacity magazines (Para Ordnance being one of the first in the late 1980s) many people find the wide grip required uncomfortable and difficult to use. 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. Tests have shown that properly prepared and calibrated 10% (by mass) gelatin at 4 degrees Celsius correlates very closely to observed performance in the muscle tissue of living, anesthetised swine. 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 distance which tissue is thrown radially outward due to "splash." The penetration represents how far into the tissue the bullet will ultimately penetrate.
Unfortunately, gelatin is a poor medium for evaluating actual effectiveness. The observed "tissue splash," usually referred to as "temporary cavitation," is not an indication of terminal performance in an animal, as gelatin has a much lower elastic limit than most living tissues; a force which tears a gelatin block in half may result in nothing more than slight bruising if applied to living flesh.
Penetration figures may not be accurate. Many testers do not calibrate their gelatin. The standard calibration is 85 mm of penetration when shot by a standard .177 caliber steel bb traveling at 180 m/s (590 ft/s). Uncalibrated gelatin may show a variance of up to + or - 50% from calibrated gelatin. Further, animals' skin resists penetration much more than the muscle tissue which gelatin simulates. Human skin tissue on the torso resists penetration as much as 50 mm (2 in) of muscle, and horses' skin is the equivalent of approximately 200 mm (7.9 in).
For a quick incapacitation, a hit to a vital, blood-bearing organ or the central nervous system 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 which penetrates deeper is required; this can be accomplished by either limiting expansion (2 times the original width is often regarded as ideal), or by using a more powerful cartridge. For hunting bear, yet more penetration is required. The pattern is, of course, that the larger the animal, the deeper its vital organs will be located, and therefore a firearm, cartridge, and bullet type should be chosen that will be able to reach the vital organs and kill humanely.
For dangerous game especially, deep penetration depth is critical; the reason for this is that the shooter cannot always choose their shots. If a hunter finds himself staring at a deer's hindquarters, it is very unlikely that he or she will choose to fire at that deer anyway, in the hopes that their bullet will be able to reach a vital organ through several layers of muscle and gut. The better choice in that scenario would be to wait until the deer decides to turn around. A lion, however, may decide to charge at a person other than the shooter, presenting a much less than optimal shooting angle.
To hit the vital organs on a large game animal requires penetrating the thick fat and muscle tissue surrounding the chest cavity, and quite often bone as well. A hard, nondeforming bullet is often chosen, though many modern rifle calibers are quite capable of killing 1,000 lb (450 kg) elk and similar-sized animals with a deforming bullet; even the venerable .30-06 is up to the task, with a powerful enough load. Elephant hunters normally attempt to shoot for the brain, which is much smaller than the size of the elephant's head, and so must be targeted quite precisely, and require a firearm and bullet capable of punching through a foot (300 mm) or more of tough, albeit hollow, bone and reaching the brain.
The rules of engagement for non-military use of firearms usually require that a life, or in some jurisdictions, property, must 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 inexperienced shooters, and the average defensive shooter 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.
Humans walk upright and present relatively unprotected vital organ targets from some angles, and have substantially thinner skin, so the bare minimum penetration is lower than for deer. Cross-torso shots and shots which must first penetrate an arm are relatively common in defensive shooting scenarios, however.
Bullets for use on humans are usually designed to comply with the FBI's penetration requirement of 12 to 18 inches (30 to 46 cm), which is based on the IWBA's requirement of 12.5 to 14 inches (32 to 36 cm). This is to ensure that the bullet can reach a vital blood-bearing organ or central nervous system structure from most angles. Frangible rounds, while they are sold for defensive purposes, are not well suited for the role, as they generally penetrate less than 10 inches (25 cm), and are therefore prone to failure when they must pass through nonvital tissues, such as a hand or arm. When they work, they work very well, but when they fail, they tend to fail badly.
Hollowpoint bullets normally expand most when at their highest velocity; that is, when entering the target. As they expand, they slow. Hollowpoint bullets may not expand when they strike sheet metal, glass, or bulky clothing before the target. These preliminary obstacles can either fill the hollowpoint cavity or deform the lips of the cavity. Either of these effects can prevent the high internal hydraulic pressure necessary to make the hollowpoint round expand.
For in-depth information on the mechanisms (and misconceptions) by which bullets incapacitate living targets, see the article on stopping power.
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, or destroy an enemy bunker. Different purposes of course require different projectile designs.
Many large calibre projectiles are filled with a high explosive which, when detonated, shatters the shell casing, producing thousands of high velocity fragments and an accompanying sharply rising blast overpressure. Others are more rarely used to release chemical or biological agents either on impact or when over the target area; designing an appropriate fuse is a difficult task which lies outside the realm of terminal ballistics.
Other large calibre projectiles use bomblets (sub-munitions), which are released by the carrier projectile at a required height or time above their target. For US artillery ammunition these projectiles are called Dual-Purpose Improved Conventional Munition (DPICM), a 155 mm M864 DPICM projectile for example contains a total of 72 shaped charge fragmentation bomblets. The use of multiple bomblets over a single HE projectile allows for a denser and less wasteful fragmentation field to be produced. If a bomblet strikes an armoured vehicle there is also a chance that the shaped charge will (if used) penetrate and disable the vehicle. A negative factor in their use is that any bomblets that fail to function go on to litter the battle field in a highly sensitive and lethal state, causing casualties long after the cessation of conflict. International conventions tend to forbid or restrict the use of this type of projectile.
Some anti-armour projectiles use what is known as a shaped charge to defeat their target. Shaped charges have been used ever since it was discovered that a block of high explosives with letters engraved in it created perfect impressions of those letters when detonated against a piece of metal. A shaped charge is an explosive charge with a hollow lined cavity at one end and a detonator at the other. They operate by the detonating high explosive collapsing the (often copper) liner into itself. Some of the collapsing liner goes on to form a constantly stretching jet of material travelling at hypersonic speed. When detonated at the correct standoff to the armour, the jet violently forces its way through the target's armour. Contrary to popular belief, the jet of a copper lined shaped charge is not molten, although it is heated to about 500 °C. This misconception is due to the metal's fluid-like behaviour, which is caused by the massive pressures produced during the explosives detonation causing the metal to flow plastically. When used in the anti-tank role, a projectile that uses a shaped charge warhead is known by the acronym HEAT (high explosive anti-tank).
Shaped charges can be defended against by the use of explosive reactive armour (ERA), or complex composite armour arrays. ERA uses a high explosive sandwiched between two, relatively thin, (normally) metallic plates. The explosive is detonated when struck by the shaped charge’s jet, the detonating explosive sandwich forces the two plates apart, lowering the jets’ penetration by interfering with, and disrupting it. A disadvantage of using ERA is that each plate can protect against a single strike, and the resulting explosion can be extremely dangerous to nearby personnel and lightly armoured structures.
Tank fired HEAT projectiles are slowly being replaced for the attack of heavy armour by so-called "kinetic energy" penetrators. Ironically, it is the most primitive (in-shape) projectiles that are hardest to defend against. A KE penetrator requires an enormous thickness of steel, or a complex armour array to protect against. They also produce a much larger diameter hole in comparison to a shaped charge and hence produce a far more extensive behind armour effect. KE penetrators are most effective when constructed of a dense tough material which is formed into a long, narrow, arrow/dart like projectile. Tungsten and depleted uranium alloys are often used as the penetrator material. The length of the penetrator is limited by the ability of the penetrator to withstand launch forces whilst in the bore and shear forces along its length at impact.
The study of projectile impacts with velocities greater than several kilometres per second is an area of active research. Such impacts are not yet used in military situations, but can arise from meteorite 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; light gas guns are currently the most common techniques for producing such speeds, although linear motors, railguns and ram accelerators are also possibilities undergoing active research.
See also Kinetic energy penetrator.