[G.Mason]

"I remember reading that in order to shorten the barrel of a gun you had to licensed by the BATF as a firearms manufacture (gunsmith). Correct me if I'm wrong."

It is my understanding that BATF prohibits sawing off shotgun barrels shorter than 18 inches, or rifle barrel shorter than 16 inches.

That is my understanding. Others may have further information.

[antaresequity]

seems to me that you could significcently shorten the form factor of a shotgun by both shortening the barrell and casting as a bullpup.

If the chamber was in the stock to an exent, you could get a few less inches out of the overall length.