Can a shotgun barrel be cut then threaded?

i was wondering if it would be legal and possible to take a rem 870 or moss berg 500 shotgun, chop the barrel to 10in then weld a tec9 styled front end back onto it to give it the 8 inch. or how about this a 10barrel with a horn type barrel like the old shotguns. so the horn adds another 8 in . im thinking of designing my own type of gun. 100% ideas. i just wonder of the legal issue with custom guns. thanks.