Think of a pump shotgun, for instance - when you fire, you pull the pump mechanism back, which ejects a shell, and another one pops into the port, then you slide it back up, which slides the next shell into the barrel. A lot of things can go wrong mechanically there.

All different types of guns function differently, but they all have some kind of mechanism like that, except maybe revolver? I don't know, I'm not a gun nut, I only have a pump shotgun haha. In any case, you have to fix the problem before you can start shooting again, which is probably a huge pain if you haven't shot a lot of rounds with said gun. That's why I think people are so impressed - he's young, but has apparently trained himself as well as or better than lots of law enforcement veterans. I mean, I still can't believe he shot the ARM THAT WAS HOLDING THE GUN of the "medic."