FMJ

A Full Metal Jacket, or FMJ, is by far the most common bullet type. Common design has a copper jacket encasing a large lead core. This bullet type rather deforms depending on point of impact and obstructions, rather than expands ("mushrooming").

Quick comparison of JHP, FMJ and AP ammunition (revise this later, nice table-esque)

Jacketed Hollowpoint: low penetration, high soft tissue damage, large permanent wound channel, rare

Full metal jacket: mediocre penetration, mediocre damage, mediocre permanent wound channel, very common

Armor piercing: excellent penetration (depends on armor), mediocre damage, relatively small wound channel (but hey, at least the target is losing blood and feeling that punishment you delivered), very rare

This and it's fair performance against both flesh and armor make it a popular choice with gunslingers. It is used by the Hunting Rifle, Sniper Rifle, and the .223 Pistol.