I don't see how that good guy scenario is supposed to play out. If the bad guy finds a crowd, he can get off 20 rounds, and kill a large number of people, before the good guy can do anything. So having good guys with guns can limit the damage to 10 victims. Unless the bad guy can get a bigger clip.
I doubt detachable magazine capacity matters. One can always carry more magazines and with practice, changing one is not time consuming. Civilians on the other hand are unlikely to carry lots of magazines so standard capacity magazines matter more for them.
Most spree shooters stop one way or another once armed resistance presents itself. Fortunately such incidents are rare despite what mass media would have us believe but this also means that it is difficult to determine what affect a good guy with a firearm will have versus the effect of law enforcement which is well known. There is an added complication in that if a civilian successfully stops a spree shooter, then there could be no mass shooting in which case they did not stop one. A similar issue crops up measuring DGUs (defensive gun uses) when it only counts if the suspect is shot or killed.
The incident at the Clackamas Town Center comes to mind:
http://en.wikipedia.org/wiki/C...