This one does not go away. 922(r) remains in effect.Wait a few months!
There is basically nothing you can do (short of making it a bolt action) to put the SHOTGUN into pistol grip configuration.
This is where so much of the confusion lays (in my mind). definition of an assault weapon seems to allow up to 10 rounds so the number 5 is irrelevant. But modding the shotgun is where 922(r) may be applied as it speaks about assembling firearms that otherwise must not be imported. That's why modding is done in such a way as to keep the us parts count up. Actually, you are looking to have no more than 10 parts from oversees on the gun. There is a list of what is considered to be a "part" (ie, stock, receiver, muzzle device, mag baseplate, pistol grip, forearm, trigger guard etc)But it only holds five rounds doesn't it?