Possible Its not cycling properly. The action is coming open before the shot has left the barrel. He has likely monkeyed with it some manner to make it "cycle faster" or some craziness like using wd40 or another flamable substanve as a lubricant. At some point its going to go kaboom and then he will be all over youtube and gun forums claiming benelli m4 are junk.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...