Math and the Zombie Apocalypse

The weirdest part is that Frank is a girl

But why is that necessary information?

I just wanted to add that even though the magazine could be emptied in 2.25 seconds, that would mean she is in full automatic mode and there's no way anyone could be accurate enough to hit all those zombies twice in the head with the bullets leaving that gun at around 13 per second. It would take quite a bit longer to account for aiming in between each zombie after a quick double tap. I'm not here for the math, I'm here for the realism in case you find yourselves in this situation and expect to be done in 8 seconds.