The Gears of War Lancer replica is certainly cool to pose with and the Nerf version is definitely... cute. But neither packs the actual killing power of the real-life version with working chainsaw attachment.
A few enterprising gun enthusiasts have gone the extra mile for their bayonets, strapping on 14 inches of chainsaw for proper eviscerating. They call it the "Firearm Mounted Anti Zombie Device" but we know it better as the standard issue Lancer from Epic Games' third-person shooter.
Yes, it works, as proven by the video of the thing in pumpkin-slaying action.
Here is just one of a half-dozen videos of these things laying waste to squashes and tree limbs. We'd definitely recommend not trying any of this at home, especially if you don't have biceps the thickness of Marcus Fenix's.
More videos and pics of folks in "Get 'R' Done" hats at the AR15 forums.