The full metal jacket (FMJ) or full metal case (FMC) bullet is also known as "hardball." The FMJ is almost entirely encased in a jacket of a reasonably malleable metal (usually copper) harder than the bullet's core (which is usually composed of soft lead).
Full metal jacket bullets offer good penetration because of their resistance to deformation. Because they do not readily deform nor fragment, FMJ is the only type of bullet allowed under common "rules of warfare" for shooting people.
FMJ bullets come in many shapes, but the most common shape of FMJ handgun bullet is round nose.
FMJ bullets provide poor results when used for hunting or self-defense, because of their tendency to penetrate while causing relatively little tissue damage.
- Russ Chastain