Swallow your morning coffee before you look at the following pics.  Enter the Metrogun:

As seen above, this 32″ beast screws into the choke tube threads of most 12 gauge shotguns, and according to the FAQ their website it does not considered an NFA item by the ATF so no paperwork is required.

More info and pics over at the Metrogun website.  Price not listed.

Commenter Farm.Dad on WeerdWorld said it best:

 I suspect if you shot down 6 miles of sewer pipe not much report would come out the other end either LOL I cannot imagine adding 32 inches to a normal shotgun barrel and trying to swing it on a target .

hahah yep.

If you end up buying a Metrogun barrel, just make sure you don’t go nuts and mistake an average plane for the Batwing.

I’m gonna stunt on Metrogun and get to work on a 64″ version with a unicycle as a monopod.  Concept mock-up: