I've got a Colt AR-15 that came with A2 furniture on it.
I know darn well that your average human skull would come apart well before the rifle would in a "percussive stress testing" exercise.
Now that it has a Magpul telescoping stock on it, it's a bit more "fragile", but still way tougher than my face, or anyone else's.
And any plastic/metallic object in your hands is better to strike someone with than bare hands. It extends your reach, it increases the mass of the swing, and is harder than your own flesh and bones. And it's a good reflex to learn for CQB/MOUT where being thrust into hand-to-hand might be a surprise to the combatants on both sides of the war. Even if you've got ammo, you might not be able to bring the muzzle to bear.
Just because an M4 is not as good a poleaxe as an M14, a Garand, or Springfield does not mean it's not a good idea to get the muscle-memory to smack someone with it in unforeseen circumstances.