But the feds only have jurisdiction to have a law about something if it is in the enumerated powers of the Constitution. To justify regulation of guns they have to fall back on either the Commerce Clause or the General Welfare Clause. If the feds don’t get their authority from one of those clauses, then the 10th Amendment means the rights are reserved to the people or to the states. So before the Supremacy Clause can even be in effect, the feds have to have express Constitional authorization to MAKE LAWS on that particular issue; otherwise the 10th Amendment specifically says that the states and/or people have the rights and the feds have no say whatsoever.
All true.
All of the federal gun laws (including, for example, the law banning gun possession by felons) actually prohibit only the possession of a gun "in or affecting interstate commerce." The Supreme Court has held that this requirement is met if the gun ever moved in interstate commerce, at any time, even if that was years before the defendant possessed it. There are also lower court cases (I'm not sure if this issue ever got to SCOTUS) which have held that it is enough if the gun was manufactured from components originating in more than one state.