I don't know about MW3, but in Black Ops there are different multipliers to the neck and upper torso versus the stomach and limbs. With most guns only the head makes a difference versus a hit anywhere else, but with the sniper rifles in particular a hit to the neck or upper torso might also be as deadly as to the head. All sniper rifles are a one shot kill to the head by default, but some especially the L96A1 are a one shot kill to I think anywhere on the body except the limbs... and that is why the L96A1 is probably the most popular sniper rifle you will encounter in Black Ops. It is a bolt action rifle and therefore has the slowest rate of fire of any gun, but it is the easiest sniper rifle to get a one hit kill with because you pretty much only just have to hit the enemy anywhere and you will kill him (unless its to the limbs, through cover, or if they have second chance).