Legal Question in Constitutional Law in District of Columbia

That's the best reference I can find, too. You may also try following the links at http://www.law.cornell.edu/topn/militia_act_of_1903.

Some people claim both that the Dick Act forbids gun-control laws and that it somehow can never be repealed. Neither of these claims is true.

The Dick Act, which is also called the Militia Act of 1903, dealt only with the structure of the National Guard

The Act is just legislation and not a constitutional amendment. It has no special immunity from being repealed or modified. Such immunity is not even possible. In the federal government, legislation cannot forbid other legislation in the future. If a law is passed today that contradicts a law passed years ago, then the new law prevails. Only a constitutional amendment -- or a Supreme Court opinion interpreting the existing text of the Constitution -- could bar particular types of future legislation. But even constitutional provisions can be changed by further amendments.

Federal legislation does trump contrary state laws, so if Congress and the President ever forbid gun-control laws at the state level then those laws will have to yield. (It's possible that such a federal law would be unconstitutional, but that would depend upon precisely what it says.) But even if that happens, the federal government could repeal the ban and/or enact contrary laws later.