Quite simply, the BTAF(sp?) doesnt want you having one. if you want one, buy the 18" and cut the barrel down, pay the $200 tax(youll have to anyway) ect. Im not sure if you also need a class 3 liscense like you do for Auto weapons. To be honest, between the $ and the Govt watching you EXTREMELY closely, its not worth it. stick with 18". If you ever NEED a 14" (i dont see a reason for 4") you can hacksaw it.