In this type of discussion, I think it's helpful to imagine real events, like Virginia Tech, Columbine or the Colorado mega-church. Any answer to the original question will sound awkward because the shooter has a huge advantage. Not only does the guy have a gun, but he also has the advantage of surprise.

That being said, to answer the original question, I would throw myself quickly and violently. The shooter would not expect it. I would try to grab something as a shield and assume a low stance, like a defensive end rushing a quarterback. Rushing in combination with using pepper spray may be quite effective. I can recall a case where two students rushed and subdued the shooter. Of course, the media doesn't celebrate the case because nobody got shot. If I can find the case, I'll post it.

My answer is no more absurd than running away if you think about it. It's far too easy to imagine a clear, undisturbed path to the door. In contrast, I imagine a traffic jam where 100 people are just as terrified as you and trying to fit through a 4 foot opening at the same time. Generally, if you take away the ability to use a firearm against somebody who has a gun AND surprise, then there's no reassuring answer. Ideally, the innocent should be allowed to defend themselves with a firearm, as in the Colorado mega-church case.