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Like our tummies, many bullets are capable of expanding. Consider three of the most common designs, full metal jacket (FMJ), softpoint (SP), and hollowpoint (HP). Regardless of the design, they all depend on the same mechanism to expand, pressure.

As a bullet travels through something, be it air, water, tissue, gelatin, or whatever, the bullet's nose feels a pressure trying to slow the bullet. If this pressure, the stagnation pressure (Ps), pushes hard enough against the bullet, the bullet will expand.

Ps = density x velocity x velocity / 2

This pressure is greater in denser media, and the pressure is a function of velocity squared. This explains why a bullet may expand when fired into water, but not the air, as water is about 1000 times denser than air. The design of the bullet dictates the velocity at which it will expand. A bullet that expands at 1000 fps may not expand at 800fps.

The difference between FMJ's and SP's is that the exposed lead nose of the SP places lead, which is much weaker than the FMJ's metal casing over its nose, against the pressure, thereby requiring less pressure to expand the SP. The difference between SP's and HP's is that the hollow nose of the HP gives the pressure a mechanical advantage so that the HP takes less pressure than does the SP to expand.

More importantly, this expression of "Ps" explains why a bullet fired into gelatin usually expands in the first two or three inches, or not at all. Why? Because the pressure on the bullet's nose that causes it to expand is greatest as it first enters the gelatin. There is no such thing as a "slow opening hollowpoint".
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