I'm tired after a long day on the road doing pest control stuff, so instead of attempting to write a whole bunch, here's a pic that explains why the obsession over firearm magazine size is dumb:
I think the people who buy into this crap must think that a shooter can't just... oh, I don't know... carry more than one magazine?!
Let's say there were such a thing as a 3,000,000 round magazine. It wouldn't do the shooter much good if he/she fired a few rounds off and was then killed when someone fired back; the remaining rounds can't fire themselves. Take firearms away from good people who would attack the shooter, reduce the number of people who can fire back, and the shooter can use up that whole magazine...
(by the way - I have no idea who made this pic, I found it on Facebook. Credit to whomever it was, don't credit it to me if you decide to share it from here.)