While working at the Atterbury range yesterday a strange thing happened. Two shooters called me over to ask me a question. They had a box of .40 cal ammo ( 50 rounds in styrofoam seperator )sitting on the shooting bench. One of the rounds exploded in the box. The styrofoam was scattered about and we found the bullet about 6 feet behind the shooters on the concrete. Could not find the brass. The box of ammo had nothing dropped on it but an ejected case may have landed on the ammo. I don't think that would cause the round to go off. I didn't have an answer for them. Any of you folks have any ideas? Thanks in advance.