The Constitution allowed slavery, for instance, and no vote for women.
It did no such thing, it simply reserved such matters to the States, per the 10th Amendment. The 14th and 19th Amendments changed that of course. The 14th was actually intended by its drafters to be interpreted more broadly than it has been, in theory it should have immediately applied the Bill of Rights against the States (including the 2nd Amendment) but SCOTUS neutered it and it has instead taken the better part of a century and a half to get most of the Bill of Rights applied against the States.
Incidentally, the established process of amending the Constitution (Article V) is available for gun control proponents to take advantage of if they think they can actually win a debate on the merits of the issue. All you need to do is convince 2/3rd's of Congress and 3/4ths of the State Legislatures to sign off on a repeal or amendment of the 2nd Amendment. Best of luck with that. :)