Glaser Safety Slug

The Glaser Safety Slug is a frangible bullet. In that, it is manufactured to ricochet less, have comprable stopping-power with standard bullets, and yet not puncture the hull of an aircraft (No ugly de-compression worries.)

Incidentally, the Glaser Safety Slug was utilised by Federal Sky Marshals in the 70s and 80s, specifically because of these properties.

A Glaser Safety Slug works as follows. The bullet consists of a fragile casing (sometimes pre-scored) which contains within it a quantity of lead shot, usually in a suspension of liquid Teflon. The shot and Teflon provide mass for the retention of kinetic energy (momentum); when the bullet strikes a soft surface (such as a person) the energy from the shot is transferred much like a normal bullet since flesh isn't typically solid enough to break the casing. If, however, the bullet strikes a hard surface (such as, in this case, the skin or window of an airplane) the casing breaks, and the kinetic energy carried by the shot and the Teflon is dispersed in all directions. This (hopefully) would be enough to prevent the bullet from penetrating the hardened surface.

One of the scenarios for use of the Glaser Safety Slug (and more in general, of frangible rounds) is urban combat and home defense in the USA.
Why in the USA? Because the prevalent way of building houses in the United States employs walls that are unable to stop most solid bullets.
A .45 handgun bullet (let alone a rifle bullet) will easily penetrate a drywall partition or a plywood door - this makes the use of solid rounds very dangerous; if you miss the hypothetical criminal, you may end up killing the invisible innocent bystander in the next apartment (or house, as the case may be).

birdshot ammunition for shotguns is also used for the same reasons; it will stop (and kill) a person at close range, but it is not very good at penetrating drywall, and it loses energy quite rapidly over distance (due to air resistance).