Add new Shoulder Stock Instead of Pistol Grip

Mark, Can't do that unless you register this firearm as SBR and send in the paperwork and buy $200 tax stamp. This firearm cannot legally be modified without filing your NFA paperwork with the ATF. This is not a shotgun, it is classified by ATF as a firea

Mark, no Sir. The classification would then change and the firearm would become an NFA firearm needing a $200 Tax Stamp and NFA Registration.