Jack is right to point out that the privileges or immunities clause of the 14th amendment was indeed, as Senator Howard at the time explained, intended to incorporate Corfield's partial listing of natural rights as well as the Bill of Rights, including the Second Amendment, as against the States declaring the body of these rights to be the true deposit of U.S. citizenship. Were the Slaughterhouse Cases and its misinterpretation of the privileges or immunities clause to be overturned, I concede that the people in the several states would no longer have an unfettered legislative ability under the 10th amendment "either to broadly protect individual rights of gun ownership or the opposite," at least insofar as "the opposite" -- that is, restriction would need to comport with the incorporated Second Amendment.
It is not clear to me however how the 14th amendment is relevant to the scope of Congress's authority over the District of Columbia, or the District's power which it has by delegation. As I mentioned in my earlier post, Congress is directly limited by the Second Amendment in terms of its regulation of the several states, but is it contended that there is a reverse incorporation of the 14th amendment privileges or immunities clause as against the Congress in its governance of the District of Columbia? If that is what is being contended, more than Senator Howard's Corfield and Bill of Rights reference in the legislative history would be needed to establish that fact and what it means -- at least for me. For now, as I see it, with or without the 14th amendment and the proper interpretation of the privileges or immunities clause, the District of Columbia may ban handguns or do something less.