Isn't it obvious? There was a struggle for the gun and it went off accidentally.
I'm not sure of that, although is not unreasonable speculation. On the side that the firing was a deliberate act by Zimmerman, we have Zimmerman saying "I shot him" instead of something like "the gun just went off."
The purpose of my post was to provide the evidence outside of what the coroner said, to narrow the range of distance between the muzzle and Martin. Because there is no witness to the moment of the shot, and at least Cutcher and her roomie Selma have said there was no fighting just before the shot rang out (after they said they couldn't hear fighting at all), the prosecutor and maybe investigator Serino though it possible that Martin had separated himself from Zimmerman before being shot. If the shot is from a foot and half, it makes that "separation before shot" scenario possible. No so much when the distance is 3 inches.