I think (not sure ) if it will fire a shot gun shell then its a shot gun and needs to be 18" however there are shot shell loads for many calibure of pistols and rifles. I do that guns like Hubles SGFH get away with being legal due to the abiltiy to still fre a factory shell and therefore are not classifed as a destuctive device ( a rifle over 50 cal) even though it will shoot a brass cased 4" or so long rifle looking cartrige with the bullet seated out exactly like a rifle.
Hmm... if you rebuild it on a bent blank you don't have to declare it one thing or another. If it's loaded with .410s it's a shotgun; if it's loaded with .444s it's a rifle. How can anyone prove otherwise?