Couple things:
Federal:
Legal rifle barrel length 16 inches or greater from face of breech the barrel to the tip.

Shotgun has to be 18 inches or greater.

In MA a shotgun needs only meet the federal 18 inches or more.
MGL 140-121
""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."

Now for the reality, since no one wants to be on the wrong side of the law barrels are often a little longer...by .5 inches in a lot of cases or even more. I know of a few companies that sell 16.1 inch barrels but goodness knows that can be a bit of an issue if it's a blank that needs a contour and crowning and reaming.

Now as to tax stamps and NFA guns:
Any individual can make short barreled shotgun...BUT you must do the paperwork FIRST, get it approved and live in a state where it is legal. For a short barreled shotgun (SBS) the MAKING tax is $200, the transfer tax is $5. This applies to individuals and not 07 FFL's who pay their SOT.
Moving on MD is a very NFA friendly state on paper, as you can own ANYTHING as an individual. That being said there is a State permit required but a few states have that.
As to ATF paperwork, if you own or buy a gun, you are part of the ATF paperwork system...even with the new NICS regulations...the 4473 still exists...you are part of the machine...no matter what.