Sometimes the ATF gets caught up in its arcane and absurd rules and delineations of guns and gun types, etc. that result in loopholes, etc. I believe one such gun is the Mossberg Shockwave shotgun with bird’s head pistol grip and ~ 14 barrel.
It’s not classified as a short-barreled rifle, shotgun, pistol, AOW, etc. just some other kind of weapon. But the minute you take off that manufacturer’s bird’s head grip you turn it into a rifle/pistol that can fall into their other ridiculous rules.