The argument is to create gun and ammunition manufacturers that are out of federal jurisdiction from start to finish, so are immune to the Interstate Commerce Clause. Instead they are regulated by state laws only.
In effect, this also means that they are immune from any federal gun control law, as there is no entry point for federal authority.
This is a major step to revitalize the idea that there is federal jurisdiction, and there is state jurisdiction, and the feds have no authority in state jurisdiction, unlike the huge number of federal redundant laws that exist today, that allows the feds to inject themselves into state authorities.
FDR used the Interstate Commerce Clause to get the feds involved with government at all levels in the US. Then LBJ used the General Welfare Clause to create the welfare state. If the SCOTUS can knock these two abuses down, then much of the federal government has no constitutional authority to exist.
Again - what gun manufacturer is going to set up a factory and only sell guns and ammo within a single state?! :-) I understand the constitutional angle but the idea is absurd.