I'm no gun expert, but I think that pistols in ArmA 3 are quite under-powered.
Even when I try to shoot civilian (for testing purposes, of course!) that is wearing absolutely no armor, it usually takes four or more rounds to kill him.
When I have to deal with armored enemies, the handgun is completely useless. It takes at least half of magazine to kill him, but usually I have to spend the whole magazine. This happens when I'm really close to the target, like under ten meters from him. When I'm further than 10 meters, it seems that the shots aren't even hitting him (even when he doesn't move at all).