IF (and I have absolutely zero idea if this would even work) it were possible to build a shotgun upper for an AR15 (or AR10, if the 15 was too small, which I suspect it would be), would it be legal to do so?

Or would it run afoul of some asinine "destructive devices" rule?

Lol I always have crazy ideas like that too. Anyways wouldn't recomend even beggining a project like that without years and years of exp. But incase you haven't seen them a Conveted Saiga 12 is basically an AK-47 that is converted to shoot 12G. I have one with several 10round AGP mags and soon I will be getting one of Mike Davidson's Drum mags (hopefully pretty soon!) And yes the debate about if the gun will go Destructive Device once the drum mags comes it is also a pretty common one. Check out Saiga-12.com if you haven't already it's the best shotgun thread EVER. Hope this helps a bit. Micah

A company in Turkey beat you to it. Not improted into the US though AFAIK.

ETA: Here it is in 410: AR shotgun