Ah, the riddle that is shorty shotties and NFA compliance

By Chris Eger

Quick, which of these are 10 years in prison if not transferred with a stamp and why? (Photo: TXMGO)
Texas Machine Gun & Ordnance posted a handy reference to explain some of the peculiarities that are the National Firearms Act and guns that fire shot.
Adopted in 1934 and interpreted variously since then, the NFA’s definitions on Title II Any Other Weapons (AOWs) and Short-Barreled Shotguns (SBSs) are a bit cut and dry when it comes to barrel and overall lengths. However, when you get into the weeds of the just what is the difference between a regular Title I firearm (such as the Remington Tac-14 or Mossberg Shockwave), a pistol-grip shotgun, or an SBS or AOW, it can get confusing in a squre-is-a-rectangle-but-a-rectangle-is-not-a-square type of way.
From TXMGO:
While assembling some Pistol Grip Only Shotguns today, it occurred to us how silly the NFA can be.
These are all the same guns, from the same factory, came from the same distributor in the same shipment, and the bottom 3 are ll fitted with the same 14″ barrel.
1. The gun comes in as a regular Title 1, Pistol Grip Shotgun.
2. We add a 14″ barrel to it, and it becomes an NFA regulated Any

Source: Guns.com