Usually there's the butt-swing to the chin, reverse-draw on the forehead, and pile-driving it on the poor SOB you've just floored.

Center of balance is usually just in front of the bolt, so baseball-swinging [i]that[/i] part of the rifle would do the most damage, and the barrel can usually be considered a 3/4" solid chunk of re-bar with a tiny lil hole running down it. [i]Axe-handles aren't even built that sturdy.[/i].
-Biggest mistake a person can make baseball-swinging is to strike with the very end of the buttstock, putting insane stress on the semi-pistol grip, usually breaking it there. (in which case, you've still got a club, no sense to stop swinging now; it's all you've got left)

All that said, getting swiped upside the head with the muzzle likely isn't going to be pleasant, either. Especially with something sharp, pointy, and heavy lugged onto it.

[i]Throwing the rifle[/i] may also be a viable tactic. But so is killing a soldier with a cobblestone-brick.

*Picks up a P-mag and wonders if he can bash a skull in with it*