Full Metal Jacket It is a movie, but as that has been covered I will give it's original meaning.
FMJ is Full metal jacket. A bullet that has a copper (or some other metal) covering or "jacket" over its lead core. This is done so that the bullet has more penetration and won't pancake on impact. Note some bullets have no metal jacket. The standard .22 caliber is an example. A hollow point bullet has a hollow tip that makes it a highly lethal manstopper because it will pancake inside the body causing more damage to organs. Theese such bullets are outlawed by the Geneva Convention.
*Note FMJ bullets are not the same as armor peircing bullets.
Why on earth would any sane 13 yr old know this?