Hand guns are difficult to be accurate with if you’ve never shot one, and still not easy if you have. Nobody is going to fire one for the first time and be good at it, but within 10-15’ the difference is probably just if the shot was lethal or not. Anything with a longer barrel like a shotgun or rifle, as long as you’re pointing in the general direction it’s as simple as pulling the trigger. The recoil is easier to deal with and it’s easier to aim. Loading could take a second to figure out, but that’s something that can be worked out before ever pulling the trigger.

I think the fact that most school shootings don’t end up with dozens of kills is because it’s not super easy to be skilled with all aspects of a firearm, but it’s easy enough for quick damage.