If you make it so that after the head sustains a certain amount of damage it will explode. So the Spas12 can blow peoples heads up. Or really heavy spray.
I like the whole head eplosion thing better then the head flies off effect because its easier to tell if you got a headshot, and its more satisfying. But some people like the flying head effect for some reason, so maybe it should be an option in the setup. Explode, Fly, or 50/50.
Oh and when a head is shot off, there should be blood gushing out of the neck or something.