Are you saying that some jury of 12 citizens can tell the entire Congress that they're wrong, the shotgun is indeed useful to a militia, Congress' law is therefore unconstitutional, and the defendant is free? Unless you believe that the military never uses machine guns for anything, I can't see why you'd find Congress' apparent belief about the military uselessness of shotguns to be more credible than its apparent belief about the military uselessness of machine guns.
Well, I thought you were talking about the
Miller case which involved a shotgun less than 18". Do you want to stick with that or would you rather not discuss
Miller and change the subject to machine guns?
Your choice.