I'm no expert, but that muzzle being wider than the actual body of the gun suggests it's a mod.
Why it would have Chaos-Marine spikes on the end is beyond me, though. Back-scratcher mode?
It's a pistol muzzle standoff. Quite a few, if not all slide-based handguns have the annoying habit of getting pushed out of battery when you shove the muzzle into someone. The spike plate you see is attached from the bottom to the frame, rather than the barrel or the slide. That way, when you shove it into someone the barrel doesn't get pushed back.
Useful stuff, for people who need to press guns into the back of heads.
