Okay, first oF all, when we're talking about gun violence in the U.S. let's talk about the biggest problems first.
The biggest issue, by far the most murders, happens with semi-auto handguns. Add suicides and accidental gun deaths, semi-auto handguns are the biggest issue... by a lot.
But we don't want to address the biggest issue first. We want to address one of the smallest. High-profile public space mass shootings.
So if that's the problem we want to solve, don't generalize U.S. gun violence and then pretend you're addressing that larger issue when you're addressing a very specific cause of gun deaths that accounts for a very small fraction of total gun deaths.
So, public space mass shootings. Semi-automatic rifles are definitely the gun of choice. Partly for functional reasons, partly because these rifles are in style and the best selling firearms over the last decade or so. They are currently the F150 of firearms, more than most their owners ever need, but when you're keeping up with the bubbas it's pretty much a must have.
These semi-auto rifles have a ****-ton of fun accessories that can be clamped right on them, from red dot heads-up sights, to flashlights, to bipods, to bull-pup fore grips, and on and on and on. But the big one, the one that makes these easy for low skill shooters to do a lot of damage with in these mass shootings are the magazines with large capacities. Commonly known as hi-cap mags, these most often come in 20-round and 30-round capacities, but you can get more. If you want a lot more you can get those drum magazines.
These AR-15 rifles are pretty easy to shoot. The recoil isn't bad at all, they are light for what they are. They aren't too long so you can swing the barrel around pretty easily.
So this is top of the list for "reasonable gun control." First, they may do what California has done, and restrict the number of "assault style features" you can have. So things like collapsable stock, detachable magazine, flash suppressor, fore grip, etc. But the only one that really matters is the magazine capacity and having a detachable magazine. The rest are either just very practical features that in some cases make the gun safer to use, like the adjustable stock, which makes it so the user can properly hold the rifle, or they're mostly cosmetic but have minimal benefits that have never really come into play in these mass shootings, like a flash suppressors. The fore grip looks aggressive, but it's purely cosmetic and really just allows a person to shoot less accurately, which makes the gun less safe even when used responsibly.
So if you want to limit magazine capacity and not allow semi-auto rifles to have detachable magazines... say that.
If you don't want any semi-auto rifles at all... say that (you'll be banning a lot of legitimate hunting rifles if that's the rule)
This is a semi-auto Remington 7400 30-06. This is a hunting rifle.
But while AR-15s are the popular gun at the moment I personally don't think they are much more effective for the kinds of mass shootings we've seen than your typical semi-auto handguns. You can get pretty high capacity magazines for 9mm handguns, they are generally easier and faster to change magazines. They wouldn't work as well over medium to long distances, but most of these things aren't really happening beyond 50ft. Within 50ft I'd argue you'd be better off with 3-4 9mm same model handguns carrying 16rnds per magazine with a belt full of additional magazines.
But since these mass shootings are more about the show and the shock, 9mm handguns haven't been the weapon of choice. Stupid since they weigh a lot less, the ammo weighs a lot less, the magazines are smaller round-for-round, magazine changing is faster and easier (with moderate skill) and you're not shooting at "hard" targets so the 9mm +p is going to do everything in that situation that a .223/5.56mm would do.
But if it makes everyone happy, let's start by banning semi-autos with high capacity detachable magazines. Maybe it'll make a difference?