The reason the barrel came off was one of two reasons. One the operator of the weapon failed to adjust the headspace and timing incorrectly. Secondly (which is more likely of two) his fellow soldiers are playing a trick on him. They just placed the barrel on the weapon and did not screw it in and they loaded the weapon with blanks to prevent injury.
+3 1. tmvanatta commented 15 years ago on video Machine Gun Fail