I'd run and hide, and look for an opportunity. That's far safer. If the shooter is within punching distance of me already, that's a different story. Shooters generally know what they plan to do, and can pick an optimal location when starting so that no one can jump them. I've been a few places where mass shootings have happened (but not while I was there), and the chances of sneaking up on the shooter are near nil. The best bet is to surround them with other armed people. Also, I find that people underestimate 2 things: crowds and water. If you can push 1 person out of the way, then 1 person can push you out of the way. Then the next 20 people can trample you.