So,

The comment that RobDarken made about Cho was true.

Onlookers stood by while Cho reloaded, and then continued shooting. The thing you aren't considering JimmyDean, is that in the event of _actual_ gunfire threatening your life, you will take cover. 99% of people's Fight or Flight response is going to be 'Flight' when faced with superior weaponry, and it damn well should be (survive at all costs). So your scenario where someone stops the armed guy by intervention with a book is unrealistic on more than one front. People will flee (or die trying to) in those scenario's, and likely I would be one of them.

As far as lessening the 'effect' of a mass shooting by limiting what firearms are available/magazine sizes are available. I go back to my original argument that when someone sets themselves in motion to commit an act of violence on a large scale, they succeed regardless of the tool. I am going to go back to the Oklahoma city bombing on this one. Timothy Mcveigh decided he wanted to blow up a federal building in response to what he saw as unlawful raids on militia (he sound sane to you??). And he did. He did so by using ammonium nitrate (fertilizer) and Diesel fuel. Both of which are publicly available, and to be honest the mixture he used was published by our government (to the public) back in the 50's as a safer alternative to using dynamyte for construction/demolition crews (The nitrate/diesel mixture was much more stable than nitro glycerine). The death toll of his actions was 168 people including 19 children under the age of 6.

So here is where I break this down

You are telling me that a crazy gunman killed 20 students and 6 adult staff at sandy hook with an assault rifle, and as a result we should consider implementing a ban on magazines that hold more than 10 rounds.

I am telling you that in 1995 a crazy man killed up 19 children under the age of 6 and 149 adults without expending a single bullet.

I challenge your belief that any kind of firearms limitation/ban will lessen the death-toll or impact of any of these type of event. If anything, people will turn to alternate methods to accomplish their goal, much as Mcveigh did. When they arrested him, he had a semi automatic pistol on him fwiw.