It's simply amazing that they can crush a skull with those plastic folding stocks........
Actually, the "science" behind the softness of the walkers has been spoken about in the graphic novel. Over time, the bones of the walkers get very soft and weak and become fragile and easily broken or crushed. So, with that in mind it wouldn't be too tasking at all. to use the butt stock of an M4 on the skull of a walker.
