To avoid having almost a foot of shotgun hanging out in your rifle's muzzleblast, you would need to whack the barrel and mag tube back to about that of a Serbu Super Shorty (10" or so).
This will leave you with a 2-round capacity. Since it will be attached to a carbine with a shoulder stock, it will be an SBS. There will be a $200 "making tax" to put it on a Form 4.
You will now have added about four pounds to the weight of your nine pound carbine in order to have a weapon that is useless for anything except maybe breaching doors. I would imagine that if your line of work involved a lot of door-breaching, your employer probably would have issued you something with which to breach them.
If you just want it for a toy, well, hey, it's America. Have at it!
C Philip said:
Short barrel shotguns are an AOW and as such only have a $5 tax stamp.
No, you are wrong.
Short-barreled shotguns have a $200 tax stamp.
Smoothbore pistols are AOW's (ie. shotguns that have
never had a buttstock attached.) They have a $5
transfer tax. But they still cost a $200 "making tax" to register initially.
This would not apply here, since even if he were to obtain a bare receiver from the factory on which to build his project, the act of ading his
faux Masterkey to his carbine would be
de facto adding a buttstock.