It's worth looking at what the Second Amendment actually says.
A well regulated militia being necessary to the security of a free State, the right of the People to keep and bear arms shall not be infringed.Its meaning has been debated on three counts. First, there are additional commas in the version passed by Congress: Geoffrey Pullum recommends ignoring them. Since no one at the time seems to have troubled themselves over whether the commas should be there or not, that seems sound to me.
Second, what does "bear arms" mean? It's been argued, notably in the linguists' brief in D.C. v. Heller, that it means to carry weapons in an army or militia. The majority view of the Supreme Court prefers the more general meaning that one might expect; the minority view dissents, each side citing persuasive examples of usage to support its case. It's clear to me that the meaning of "bear arms" depended then, as it does now, on the context, which doesn't get us very far since our problem is to understand what the context might be.
Third, and to me most importantly, what is the prefatory clause about the militia doing there at all? Why not keep it snappy and just put "The right of the People..."? The linguists' brief explains that grammatically the opening clause is an 'absolute construction' which should be interpreted by prefacing "Because", and the majority view of the Supreme Court was happy to agree with that. Eugene Volokh argues powerfully that in explaining its objective the Second Amendment does not mean to restrict its scope to the satisfaction of that objective.
I suggest that another reading is possible. Consider this:
The weather being so hot, staff are permitted to take off their jackets in the office.Everyone would agree that the permission to undress applies only for the duration of the hot weather. This sort of interpretation has been considered in detail by Stump (p62), whose work is explained in less technical language by Fernald (p21). The summary seems to be that if the absolute clause is time-limited, the operative clause applies only for the limited time. So if the necessity of a well-regulated militia was envisaged not to be permanent at the time of the enacting of the Second Amendment, and if it has now lapsed, then the Amendment is now null.
So what was understood by the legislature at the time it passed the Amendment, and by the States when they ratified it? We might look at James Madison's earlier draft:
The right of the people to keep and bear arms shall not be infringed; a well armed and well regulated militia being the best security of a free country: but no person religiously scrupulous of bearing arms shall be compelled to render military service in person.It's clear that he envisaged no temporal restriction. But that's largely irrelevant. The legislature in adopting the Amendment chose not just to delete the 'Conscience Clause' but also to move the absolute clause to the beginning, thus making the meaning less clear. Why did it do that? I can think of only one reason, which is that a certain ambiguity was required to get the Amendment passed - political fudge is not a recent invention. The legislature did not agree to create an unambiguous constitutional right for the individual to keep and bear arms. Very probably many of its members would have been in favour of such a right, had they considered the question at all, but then very probably they would have been in favour of all sorts of things that are not part of the US constitution. (This is an example of the general weakness of interpretation according to original intent.)
I suggest that in this case, where the legislature has wilfully declined to make itself clear, it is the duty of the Supreme Court to consider present circumstances rather than to seek to divine what a majority understanding of the original meaning might have been in 1791. And present circumstances strongly suggest that it is not best to permit the general population to keep and bear semi-automatic weapons.