There is an 18" barrel minimum. Close the action (UNLOADED), and stick a tapemeasure down until it hits the breach face. If it is less than 18" you have to have special permits and tax stamps. Under 18" I would be actively destroying the barrel (bandsaw, cutting torch, sledgehammer ect...) because if the boys in black catch you with it and a gun that fits it, they will assume that the two go together.
Eagle