Quote:
Originally Posted by dreaming
Should the firearm never been designed or manufactured stock that allows it to be fired from the shoulder, you may find that it is legal under Federal law, as is the case if the Mauser 500 Persuader.
In this case it wouldn't be a shotgun, it would simply be a "firearm" and would be classified as an "other". These types of firearms don't have a barrel length limit, but they have an overall length limit of 26". So if you buy a new pistol-grip shotgun you can make the barrel as short as you want without registering it under the NFA so long as your overall length is 26" or more.
Check out Shockwave Technologies, they make a birds-head grip that makes a Mossberg 500/590 with a 14" barrel into a non-NFA firearm by bringing the overall length to over 26" (keep in mind that this only applies if your Mossberg has never had a stock installed on it):