[TigersEye]

Interesting issue ... and here I'm referring to shotguns which were originally made with a rear pistol grip in place of a buttstock.

So what does that make a Mossberg that originally had a long barrel that was cut off and shoulder stock replaced by a pistol grip?

(out of curiosity of course, my boat is at the dock ready to go)

[PapaBear3625]

If it was originally made as a pistol, then it's a pistol.

If it was originally made as a longarm, and you convert it to a pistol, then you have a problem.

[Elderberry]

You better take that boat ride:

As I read this. It appears to me.
You possess the evil DD

26 U.S. Code § 5845 - Definitions

For the purpose of this chapter—
(a) Firearm
The term “firearm” means
(1) a shotgun having a barrel or barrels of less than 18 inches in length;
(2) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length;