Cutting the barrel down to 18.5" on a shotgun is perfectly legal. Cutting it shorter than that is legal also provided that you have an approved $200 NFA tax stamp from the BATF in your hands at the time of the cutting, and the tax stamp is not very hard to get.
Briman good to know that, but without the stamp it would illegal right? But I was told by a gunsmith that it was illegal to alter the length of a barrel when I asked him to shorten and re crown a slug barrel for me. he stated you could lengthen a barrel I dont know how you would do that other then welding on a muzzle break LOL but it was illegal to shorten a barrel even if it was within the legal length limit. who knows........