You guys ever been duck hunting? If so, you may have seen a punt gun. Think 2-4 gauge shot gun. Of course it would take a monster to hold onto that and not have their shoulder ripped apart.
You could kill someone with that gun if you held the butt end up close to someones head and pulled the trigger. Isn't there some weapon that troops have now that emitts some ultra-sonic noise that makes you poop?
single shell. a SAW M1-A4 Machine gun {think of normandy} A 50. Cal big boy Sniper rifle. A MAWAS Rocket launcher.