you can't acquire an automatic weapon, or "machine gun", in the US without either an FFL, or buying an expensive as fuck and extremely rare automatic gun from pre-1986. You might see firearms with fire rates similar to automatic weapons as a result of illegal modifications, like that of the bump stock, but there are also less reversible modifications someone might end up doing. Anyways that's more like a theoretical, really stupid correction for me to make, because it's kind of up in the air as to whether or not automatic weapons would even be more effective if you wanted to kill a lot of people, as military doctrine generally employs them (full auto) as suppression or cover fire, making active zones of danger which enemies can't pass through or fire from, rather than for the use of killing people. Though, the military doesn't really tend to kill large unarmed groups of people, or, they prefer to do that with drone strikes, anyways. You don't really care about any of that, though, probably.

I would also like to posit that probably america has a unique combination of factors which spurn on violence. Insane amounts of wealth disparity, probably only comparable to some places in the middle east, if that, but also a sense of entitlement towards middle class living, aka the "american dream", which creates a kind of scorn and spite in the american mind when that middle class ideal is denied, or revealed as false. The way that these ideologies work is that they say that X is entitled to middle class living, that they deserve it, but that Y minority or Y oppressed group is in the way.

Also, these mass shootings, mass shootings of this specific type, tend to be relatively rare. Or at least, not as big of a problem as the media would have you believe, relative to: the vast majority of firearm violence, which primarily happens with handguns, and is related to gang violence (this category includes shootings by the police). Which is quite obviously related to poverty, and the protection of drugs as a high-value good that obviously can't be protected by the actual government. So you see a local monopoly of force evolve taking advantage of the poor in order to bring themselves to a more economically workable position, yadda yadda, I'm sure you've heard that story before. And then on top of that you have handgun suicide comprising somewhere between half and a third of all gun deaths (I can't quite remember).

All that considered, in combination with a lack of political will to get rid of guns, for somewhere around half the population, I'd probably make the prescription that you would see a better drop in violence from the legalization, or decriminalization, of drugs, universal mental healthcare, rectifying economic inequality, and of course, "common sense" gun laws, which would probably mostly apply to screenings for mental illness, primarily depression, but also conspiratorial thinking. The latter there, "common sense" gun laws, I think is agreeable to the majority of the population.