You could ban guns completely and people would manage to get them illegally from Mexico or somewhere else
Or make them.
I’ve made several firearms in my basement, using tools readily availble at Home Depot or Lowes.
In my case, they have been replicas of early period flintlock and wheelocks, but there is no reason that I could not have produced revolvers or even a submachine gun.
The WW-II British Sten gun, for example, was designed specifically to be produced in garage shops. The Jewish Zionists under the British administration of Palestine, produced those in substantial numbers. In one case, in was in the basement of a laudry shop that served British officers. The noise of the washers and dryers was used to cover up the noise of the gunshop below… while at the same time, British officers were dropping off their laundry ")
The skill set required is not all that extensive. I’m not a metal worker at all, (my degree is in COmputer Engineering

, but I was able to figure things out well enough to cut out gun parts on a band saw and grind, file and buff them as needed.
Contrast that to the chemical knowledge need to fab up most illegal drugs. And that is not even counting the special tools, chemicals and raw materials. As I mentioned the tools I needed I got a Lowes, as most of the metal too, either there or at the local scrap shop. The only ‘exotic’ material I needed was some really nice cherry and curly maple woodstock
If there was a general gun band, I would expect such gun shops turning up in every major city (minus the curly maple, but I suppose even that could be included for a price). And those shops would not be doing single guns, but mass producing them A gun would be no more difficult to obtain in any city than a bag of pot is right now.
Another interesting thing is 3D printing. For those who are unaware, it is technology that uses either plastic (at the low end) or lasers to melt metal powder (at the high end) and form objects. Just like a printer does, but in layer after layer. Feed in your design, and out comes the object, fully formed.
There is an ‘Open Source’ project that offers download object files that produce guns. If you have access to one of these machines, just feed in the file and in a few hours, you have a functional, if limited, firearm.
youtube.com/watch?v=AQ6Q3BfbVBU&feature=player_embedded
How would THAT fit into a gun ban.