You can't beat a machine gun with a chainsaw attatched to the front. If the military invented something like this, do you think we'd still have wars? Everyone would be too afraid to attack us because they'd be like "Ok we hate America let's go attack them" and then the other guy would be all "No, dude they got those chainsaw guns" and then the first guy would be like "Oh, you're right. Those chainsaw guns suck. That's not fair." and then the American guy would burst through the door in slow motion with an explosion going on behind him and be like "We heard you talking about America in a negative way" and they would be like "No we were just talking about how much we didn't hate you" and the American guy would be like "Shut up, fool!" and start cutting up everyone. I think I just wrote the script for the next Michael Bay movie (bah dum tish).
Seriously, I would not even care about the enemies, I would just run straight up to someone and cut them up. Even if I died, I just enjoyed the satisfaction of sticking a chainsaw up someone's ass.