I don't understand how using different ammo can cause the gun to fire multiple rounds. Can anyone explain this to me?
Some guns like the AR15 do have a firing pin that makes contact with the primer when the round is chambered. I've heard of cases were a round had a high primer that caused a double fire when chambered. That's the only thing I can think of ammo wise that would cause it to fire multiple rounds.
David Olofson is a U.S. Army veteran who was tried and convicted by the U.S. Federal government on the charge of "transferring a machine gun" after he lent his 20-year old AR-15 rifle to a friend, Robert Kiernicki, and it malfunctioned on a shooting range, emitting multiple rounds and immediately jamming three times in a row, which aroused suspicion that the rifle was in fact an automatic weapon.
Kiernicki testified that Olofson had told Kiernicki that the third position of the rifle's firing selector was for automatic firing, but it jammed, court records indicate. He also testified Olofson told him he had fired the weapon on the automatic setting at that same range without a problem. According to Len Savage, a weaponry expert who runs Historic Arms LLC, BATFE paid Kiernicki an undisclosed amount of money for his testimony.