Except two deep lacerations to the back of his head and the fear that he would be dead at the hands of this 6' 2 man bashing his head on the sidewalk if he couldn't get his gun. Thank God he did.
Zimmerman was taking a beating for 30-60 seconds or so. Did not go for his gun, yelled for help. While shifting his body so his head would be smashed into grass, instead of into the sidewalk, his shirt pulled up and his firearm became visible. At that point, says Zimmerman, Trayvon issued his death threat, "You're gonna die tonight." Trayvon was attempting to disarm Zimmerman so he could shoot Zimmerman. Probably both of them had hands on the pistol. If Trayvon pulls the gun toward himself, he very well could have "pulled the trigger" by his action. IOW, it may be that Zimmerman did not intend the firearm to discharge.
We don't know what he said under questioning, or why one of the investigators didn't believe the story. We don't have that affidavit to get a handle on the counter-argument.