The Remington TAC-14 is a shorty shotgun that is not subject to NFA rules. It’s made in the factory without a stock, and is therefore simply a firearm, and doesn’t require a tax stamp.
The gun above from Black Aces Tactical is a semi-automatic Remington 1100 that is cut down to similar proportions.
Because it’s semi-auto, it isn’t fully exempt, but it’s still not a short-barreled shotgun (SBS), but rather an AOW (any other weapon). That means you just need to pay a $5 fee and fill out a Form 4, and wait, of course.
The barrel is 12 inches long, the mag tube has a capacity of 4+1 rounds. Technically it could be called a Remington 1100 Shockwave, as it comes with the Shockwave grip…which is where the Mossberg 590 Shockwave gets its name, but that’s a different gun.