Mark Cleaver

I am doing my first conversion to my new Saiga 12 and after i add the folding stock, piatol grip, FCG and new hand guard, thought about cutting the barrel down and welding the door buster I purchased but, "not supposed to have an 18" shotgun"? I thought the legal length was 16". Am I wrong on this??? Haven t cut anything yet, but when I do I will remove the barrel from the receiver to be legal.