Militia laws and pre-emption in the early Republic
Norman Heath's article on militia laws and pre-emption raises an interesting point regarding original understanding. It's not so much how early Americans understood the Second Amendment, as evidence of how they didn't understand it, namely as aimed at the Congressional power over militias.
[Update: Norman Heath emailed me to point out that his thesis goes beyond issues of original understanding. Rather, we can reason (1) the question is whether the Second Amendment protects rights of individuals to arms, or rights of States to maintain a militia free of federal interference; (2) from the earliest days of the Republic, and into modern times, the courts made clear that States have no constitutional right to have their militias free of federal interference; ergo (3) the State right theory has nothing to support it, and we are left with the individual rights approach.]
The question of Federal pre-emption arises when Congress and the States both have overlapping legal powers, and under the Supremacy Clause, Federal power is superior. Suppose that on a given subject Congress passes some legislation. It doesn't expressly forbid States to do the same, but has it implicitly forbidden them to do so, by acting with the understanding that its standards will be the only ones applied? And what if the States want to enact measures that supplement the Federal ones, or differ from them in some way?
Today when we think of pre-emption we think of the Commerce Clause -- Congress regulates interstate commerce in some way, and States regulate commerce in general. But Heath points out the Court's jurisprudence in this area actually arose out of the early militia statutes. Congress had Federal militia laws, States had their own, and often there were problems -- the State commanding more than the Federal did, or having different commands or different penalties, or the President calls out the militia and the States claim that they need not comply.
The Second Amendment relevance: nowhere in that entire chain of Federal cases (and only once in the State cases, a dissent) does a court suggest that the Second Amendment has anything to do with the question. If early Americans had had any concept that the Second Amendment protected State powers over their militias, the States would surely have made it a major part of their legal argument. Instead, they seem to have ignored it.