A full metal jacket bullet is a bullet that is completely encased in a jacket. There is no exposed lead or hollowpoint. When it hits the deer it will not normally expand and will not do as much damage as an expanding bullet. Therefore less likely to make a clean kill. This is why the Geneva convention specifies full metal jacket bullets for war. They will not do as much damage when they hit a person.
You know.......I've heard it said that in war, the objective is to wound, not kill. Reason? The troops spend (lose) time gathering up their wounded. Slows them down.
"If the river sounds; somethings coming"