So regarding the Mossberg 14" non-NFA stuff:
Legal in MA? Probably not.
MGL Ch 140 S 121 said:
“Shotgun”, a weapon having a smooth bore with a barrel length equal to or greater than 18 inches with an overall length equal to or greater than 26 inches, and capable of discharging a shot or bullet for each pull of the trigger.
So, with a 14" barrel it wouldn't qualify as a "Shotgun" in MA. But:
MGL Ch 140 S 121 said:
“Sawed-off shotgun”, any weapon made from a shotgun, whether by alteration, modification or otherwise, if such weapon as modified has one or more barrels less than 18 inches in length or as modified has an overall length of less than 26 inches.
So even taking a firearm that Mossberg shipped from the factory never having had a stock on it (as is required Federally for this modification to be non-NFA), MA would probably consider it a "Sawed-off shotgun" since, as it came from Mossberg, it would qualify in MA as a "Shotgun".
Now, if Mossberg ever sold a firearm in this configuration
from the factory, then that
might be legal in MA, but IANAL. Mossberg does sell a model 590 factory SBS, however, that could likely be transferred to you on a Form 4.
In free states, barring any other local or state prohibition (which I would argue makes it not "free"), you can indeed make one of these things legally. The most important point though is that the receiver must have never had a stock on it. All those "pistol grip only" (PGO) models that are sold? Those are not "shotguns" from a Federal perspective. They are "firearms", more specifically Title I firearms, since the OAL is greater than 26" and it doesn't have a stock, so it doesn't get you into Title II registration territory.