Under current ATF rulings, its possible to create a non-NFA firearm that uses shotgun rounds (note, I did not say “shotgun” which refers to a specific legal definition). By never mounting a stock to a receiver and keeping the over-all length over 26″, Shockwave technologies Raptor grips keep a scattergun legal to own. Users who purchase these must keep the grips on, otherwise they become an AOW requiring registration.
Shockwave has the full details of the rulings (with appropriate letters from the ATF) available on their website. You can check the details and Shockwave by clicking the link here. The “Raptor” grip as its called is available for both Mossberg 500 and Remington 870 shotguns.