Ban automatic guns to start off.
"automatic" - not sure if you realize it, but every automatic in this country is HEAVILY regulated behind stamps and they cost an absolute small fortune. They are also pretty much never involved in mass shootings. They're more of a gangland modded trigger type gun.
So I presume you actually mean "semi-automatic" here... and if that's the case, that's basically everything but pump-action shot guns, lever-action & bolt rifles, and revolvers. Now... there's no shortage of any of those, but semi-auto dwarfs that in terms of sheer volume of guns in the United States. And are you saying ban them all going forward? Or ban them all retro-actively? The former would be a herculean effort. The latter, you'd likely start an actual domestic war over if you got it anywhere near actual law.
I'm just here to educate, BTW. https://www.stonekettle.com/2015/06/bang-bang-sanity.html is a fantastic read. I recommend it for both gun-owners and gun-haters alike. It's the first real-world solution I've seen that I think both could find fairly reasonable.