Terminal ballistics is the study of the behavior of a projectile when it hits its target. It is often referred to as stopping power when dealing with living targets.
There are three basic classes of projectiles:
- Those designed for maximum accuracy at varying ranges. These bullets are usually related to long range precision target shooting.
- Those designed to maximize damage to a target by penetrating as deeply as possible. These bullets are usually related to dangerous game hunting.
- Those designed to maximize damage to a target by deforming to control the depth to which the bullet penetrates. These bullets are usually related to controlled penetration and work very well in a wide variety of hunting applications, such as small, medium and large game animals.
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. Further, animals' skin resists penetration much more than the muscle tissue which gelatin simulates.
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.