I don't have a list in hand of all firearms, but I believe that certain firearms should be reserved for military or industrial use, and not for use by citizens.
I think I would have to know the context in which the NFA made those statements.
I'll give you an example: a Flak gun, which generally has to be installed on the turret of a military vehicle. Obviously, common sense tells us that it isn't necessary to address that. Personally, I certainly would not consider legalizing the use of the Flak gun for hunting grouse.