If you got rid of all the guns, what would the next madman or terrorist use to commit mass murder?
Guns. They would still use guns.
You could implement the most terrible, overreaching, violent and invasive regime that the world has ever seen. You could send agents of that state door to door, to entirely dismantle every home, dig up every yard, and react to every instance of non compliance with a swift and decisive death penalty, in an effort to remove every gun from public circulation.
At the end of it all, they would still get guns to commit their heinous crimes.
Unless you're ready to dismantle all of industrialization, all of society, thousands of years of technological advancement, to the point that people could no longer work a piece of metal, or engage in basic chemistry, you'll never make the gun go away.
At this point, it's easier than ever to simply make guns. Not even just 3d printed novelties. CNC machining is cheaper and more accessable than ever. CAD is widespread and accessible. Even the ability to learn manual machining and metal working techniques is easier than ever. There's more access to the materials, to the niche aspects of working those materials, and to the manufacturing processes specific to firearms.
The cat is thoroughly out of the bag here. The bag itself is shredded beyond repair. There's no going backwards from this point and it's pure folly to still think in the terms of the past, that this can somehow still just be reasonably gatekept, and that anyone who's interested enough couldn't just make what they want, even if they somehow couldn't just easily access it through channels outside the control of an individual regime.
It's time to look for different approaches, because traditional gun control is largely a relic of an era that no longer exists.