Mass shooters almost never plan past the "kill a bunch of people in a horrific environment" stage. Even when they do (see the Aurora shooter) they focus on tacticool bullshit rather than actual tactics. Guy's gun jammed about instantly with his super cool 100-round mag, had to toss it and go to work with a shotgun.

In related news, I'm all about high capacity mags! Yes, really. They jam constantly, total pain in the ass, and people who actual shoot guns scoff at them. LOL, I mostly use 10 and 20-round mags in my AR because they never fail. Found a nice Magpul 50-round at the thrift one day for $15. Was tempted to buy it for giggles, for just a second. Nah.

Anyway, I don't expect a jammer to be in a murderer's kit.