Article from 2010, but all the pros it lists for "home defense" also are pros for just plain killing people/being effective weapons in a mass killing.
http://www.gunsandammo.com/2012/02/1...fense-caliber/
A 6 or 7 year old absorbing a rifle round that is designed to fragment/xfer the energy into the target is why there were essentially no survivors. At least with the handgun models they mentioned he had, the bullets would have been more likely to pass through the kids. There still would have been a lot of deaths, but maybe less? The Bushmaster also appears to have a 20 to 30 round standard clip size. The Med Examiner office said most of the victims were shot more then once, so assuming an average of 2 shots per victim, he potentially only had to reload once or maybe twice max if he was a good shot (which from all evidence provided, it appears he was).