The supermajority of mass shootings aren't with an AR15, they don't fire any faster than pistols (actually, gas is usually slower than blowback with full auto too), they use a smaller cross-section (which is another discussion), they don't have non-FMJ options commonly available/used (which is the biggest issue, especially with its smaller calibers), and cause more wounding, but less death (statical reality, despite the assumptions/narratives) ... let alone jam far more often.
That's why the Media shows an AR15 silhouette for any 'assault weapon,' which is used in over 96% of homicides, ut isn't even a long rifle.
Virtually every major mass shooter that has attempted to use high capacity magazines has had them jam and the carnage ended as a result. Parkland shooter? 10 cartridge magazines ... the punk did it to prove a point, that someone well trained can do a lot more, with a lot less in 1/10th the time.