A lot of people are talking about banning Ar-15s in the US. So lets go down that road a bit. For this mental exercise we are going to ban all semi-auto rifles because the only real difference between grand dads deer rifle and the Ar-15 is how it looks.
Assume we make semi-autos LEO and military only ( BTW military don’t want em they got much better weapons), . They are still being made and shipped and since they are now banned they are going to bring lots more on the black market of the criminal underworld. So a enterprising crime syndicate will be stealing them and opening a underworld market. Actually the market is already there just gotta expand it.
So we ban the production entirely, yet other countries are still making them so same thing just going to steal them.
So lets get the whole world to just quit making them, that not going to work because if you have a little skill and some machine shop equipment you can build em from scratch yourself with the right plans, and you can’t eliminate the machining capability because it is used for all the other stuff we build.
Oh so we just ban the ammo right? except we use that ammo in all the guns that are not semi-auto.
Oh lets eliminate all guns. We are back to the whole make your own in a machine shop except now all you need is a piece of pipe and a nail to build a zip gun.
But with all guns being illegal all over the world for any reason there is no reason to manufacture ammo anymore. Except handloading is a thing using used casings. And a smart criminal enterprise would build a ammo factory to supply their underworld gun shop. And gun powder is easy to manufacture using available ingredients.
so eliminate the ingredients for gunpowder, not gonna happen but just assume it happens. Well there are many other chemicals to make a propellant with. I know of a catalyst used for a industrial glue that when dried out and ground fine burns just like gunpowder just a lot dirtier.
So as you can see no matter what you do you will never get rid of guns, just like with drugs if you make them illegal there will be those that will abide by the law and there will be those that make a fortune off the illegal sales and manufacture.
None of the above takes into account the million up millions of guns that are still out there or the fact that there are many many more options to kill large amounts of people. While there is still people there will be violent people to do violence on others.