Not only is it more complicated, it doesn't even matter.
Around 80% of mass shooters bought the guns legally. Of the 20% remaining, the majority are teenagers who used their parent's legally owned firearm.
Criminals in America have better access to firearms than they do in anywhere else in the world, with many of the guns in South America being originally purchased from a store in the United States.
This has resulted in a homicide rate that is far higher than it should be. Sort this list by homicide rate and take note of just how far before and after "United States" you have to scroll before finding a country you would consider "wealthy and stable".
As compensation for that, we're told things like "the only thing that can stop a bad guy with a gun is a good guy with a gun". But the "good guys" have been given all the guns they want and they stop exactly fuck all. It's not even close to the number of shootings they enable.
So who gives a fuck how "effective" they are on paper? In the real world, police and unarmed civilians stop more mass shooters and it doesn't require arming the mass shooters in the first place.