Davi Barker does a good survey of shooting rampages and compares the number of victims in attacks stopped by the police versus those stopped by a regular citizens.
With 14 incidents stopped by police with a total of 200 dead that’s an average of about 14.3. With 15 incidents stopped by civilians and 35 dead that’s an average of 2.3.
The first point I want to draw your attention to is that roughly half of shooting rampages end in suicide anyway. What that means is that police are not even in a position to stop most of them. Only the civilians present at the time of the shooting have any opportunity to stop those shooters. That’s probably more important than the statistic itself. In a shooting rampage, counting on the police to intervene at all is a coin flip at best.
Second, within the civilian category 10 of the 15 shootings were stopped by unarmed civilians. What’s amazing about that is that whether armed or not, when a civilian plays hero it seems to save a lot of lives.
Important lessons there. One: by the time the police get there there's good odds that the attacker will be dead by his own hand or have simply stopped. Two: Fighting back is better than freezing and dying. No matter how you're armed, it will help. Three: Carry your damn guns.
Go to the link. Read it all.