fmj rounds aren't meant to kill a person, they are just meant to put him out of action. Thus tiring out the guys that have to go out in the field, jungle, swamp etc. to get him. Wasting fuel hauling his ass back to a hospital. Tiring out the doctors who have to patch him up. Wasting valuable medical supplies and food resources keeping him alive while he can't fight. Disrupting the enemy's supply while it is under stress can do allot of damage. They were designed for war and when at war it's better to strain the war effort of the enemy rather than kill everyone. Alot harder to fix a man up to get him back on the front than it is to grab a "new" one and give him a rifle.
It'll kill a deer, he just wont flop in his tracks most likely. Same with a person.