Reports indicate that Altman was carrying a 9mm Springfield Armory XDM semi-automatic handgun. The gun has a number of built in safety features including a grip safety and a chamber load indicator. The chamber load indicator would have told Altman that there was a bullet in the chamber of the gun, ready to fire. The grip safety prevents the striker from releasing if the gun is dropped, but since Altman was holding the gun at the time of the discharge, and presumably holding down the grip safety, this safety feature was not a factor.
Reports do not indicate what Altman feared that he felt it was necessary to carry a loaded gun, with a round in the chamber, into the convenience store.