I remember hearing (I don't know if it was from a real source or rumor) that GZ's gun jammed when chambering the next round. Which points to a struggle over the gun.
If the rear of the gun's slide bumped into Zimmerman's body, that would have also interfered with the gun's cycling. If Zimmerman's on the ground trying to fire at an attacker on top of him, it's quite possible that some part of him got in the way of the slide's cycling.
Either way, the lack of cycling is indicative of Zimmerman being in a struggle, and would point towards self-defense by Zimmerman.