Actually, I haven't taken a close look, but I believe the rifle is the type with a universal rail attachment on the handguard - so you don't have to modify the weapon itself at all; just build the chainsaw in such a way that it can make use of the attachment. It's the same thing used to mount tactical lights, etc. on SWAT weapons. I believe there's a video out there where they mount the same saw on a shotgun. Hence it's pretty clear that this guy knows what he's doing. It's not a weapon so much as it is a "nice hack" (pun not intended).