They don't have to add later. The language of the law covers the named firearms and functional equivalents.
I understand the part about derivatives. We've lived through that
My point was, imagine something like Kel Tec's KSG. It's not listed, and it's not really derivative of anything on the list. If one is used in a high-profile crime, or if a politician just gets his knickers in a schnitz, they can simply add it to the list.