"Zimmerman said Martin was on top of him, slamming his head against the ground when he grabbed his gun from a holster before Martin could get it." Did you read that part ...."before Martin could get it."?
Likely, there was a scuffle for Zimmermans gun, but Martin was unable to get a hand on it before he died.
What else they know is from the autopsy report on Martin and the medical report on Zimmerman:
Two black eyes
Lacerations on the back of his head
Bloody knuckles with Zimmerman's DNA