FMJ
Full Metal Jacket: a bullet with a soft lead core fully encased in a harder metal shell.
Also: full metal jacket, ball ammo
FMJ bullets have a soft lead core wrapped in a copper or steel jacket. They penetrate well, feed reliably in semi-autos, and are cheap to manufacture — making them the standard for range/training ammunition. They’re poor for self-defense or hunting because they don’t expand on impact.