If a public mass shooter wanted the best gun for the job, they’d get something closer to a PS-90 (the civilian version of the P-90 which is a military rifle designed for urban combat).
You’re neglecting the fact that mass shooters are murderers, so close combat isn’t their thing - and the best gun for the job has already been shown - ar15-style rifles - 14!! - with drum mags and bump stocks:
On October 1, 2017, a mass shooting occurred when 64-year-old Stephen Paddock opened fire on the crowd attending the Route 91 Harvest music festival on the Las Vegas Strip in Nevada from his 32nd-floor suites in the Mandalay Bay hotel. He fired more than 1,000 rounds, killing 60 people[a] and wounding at least 413. The ensuing panic brought the total number of injured to approximately 867. About an hour later, he was found dead in his room from a self-inflicted gunshot wound.