The hollow point bullet is different from a solid lead bullet, a partially jacketed bullet, and a full metal jacketed bullet.
Hollow point ammunition, as its name suggests, has a hollow anterior tip.
The hollow point, depending on which kind of round, can improve or degrade accuracy; but the point of a hollow point — pun intended — is not flight (accuracy) ballistics, but terminal ballistics.
Terminal ballistics is the study of the action of a bullet on a living target, sometimes called an animal or human being.
The hollow point on hollow point ammunition is designed to produce a different result in terminal ballistics from other bullets.
This result is created by the way the bullet behaves when it hits a body, that is, the hollow tip moving at great speed (usually supersonic) does one or two things.
The main thing the hollow point does when it hits a body is spall, sometimes called “mushrooming”; it spreads open, making the sleek bullet that entered the body flatten out on the tip, and causing it look a little like a mushroom.
The second thing that can happen when the bullet hits a body and mushrooms is that it can fragment, that is, break into little pieces as it transits through the living tissue.
A secondary consideration for using hollow points is that you can more easily kill one person in a situation with several people without hitting more than one person accidentally.
When the hollow point bullet spalls, the increased surface area of the bullet encounters greater resistance from those living tissues, and so is more likely to stay in the body of the person being shot intentionally.
When hollow points miss the living person (or animal) and encounter other surfaces, they behave in unpredictable ways, ricocheting and-or breaking into multiple missiles.
The terminal ballistics for hollow point ammunition involves three important components: a wound track, cavitation, and fragmentation.
The wound track is the path of the bullet through living tissue.
Cavitation is the spreading and tearing of living tissue along the wound track, with two kinds of cavitation: permanent and temporary.
Permanent cavitation is the damage adjacent to the would track that remains visible after the wound is inflicted.
Temporary cavitation is a millisecond wide-spreading of tissues in the wake of the bullet that spends shock waves through a much larger area, bruising tissue and transferring energy outward.
Fragmentation — fragments leaving the wound track to create many smaller wound tracks — can be from the fragmented hollow point bullet or from bone hit by the bullet, often both.
Sometimes the hollow point, and other bullets as well, can “tumble,” meaning the bullet quits spinning laterally — the source of its flight stability — and do somersaults inside the person who is shot, which vastly expands the permanent wound track, changes the course of the bullet where it can hit more tissue, bone, and organs.
Tumbling is considered a plus for war ammunition, hollow point or not, because it destroys a great deal more tissue, bone, and organs.