How would a magazine capacity law prevent any crime?

In the wake of the Arizona shooting, the tragedy it was, a few lawmakers are trying to impose a new gun law limiting magazine capacity to ten rounds. My question is this, how would that prevent any crime? The average reload time for a untrained person is around 5 seconds with records around a second. http://www.youtube.com/watch?v=ksZqzPWm7VQ Most of these lawmakers are completely ignorant about guns and are just making the law to take away our freedoms and make some people feel better. I know there are some who say "Why do you need 17 or 30 rounds", to that I say why not. Have any of yall ever fired 30 rounds from a m-4 in fast succession, its freakin fun! And in a self defense situation I want as many rounds as I can carry, not to mention that most weapons that people conceal carry with are sub-compact and hold from 5-8 from a physical limit. I'm not trying to show political sides, I just would like to know how some people would rationalize a law that makes so little sense.