It is a copy of one of the guns used in Gears of War. You can get a chainsaw bayonet added on the rifle, and some other random attachments.
So some guy tried to reproduce the shape of the original rifle (scope bottle, pipes, etc) then added a chainsaw for the bayonet (but on the side of the gun, considering it wouldn't fit on the bayo lug). Then he took a video of him shooting it and put it on youtube for people with more common sense to shake their heads at.