Factory pistol gripped shotgun, can not be bought across state lines or by those under 21 from a dealer, private sales are OK.
On form 4473 they are not a long gun, they are not a handgun they are an other.
An other can be configured into a NFA regulated weapon called an Any Other Weapon.
A shotgun can not be configured into an AOW but would become Short Shotgun.
So you can put a pistol grip on a shotgun, and it is still a shotgun.