I think the magazine being reference is:
These magazines are prone to jamming, especially when loaded incorrectly. In fact, most weapon jams are magazine-related. Thankfully, this maniac appears to be a weapons novice who did not know to perform immediate action on a jammed rifle. As for the belief that higher capacity magazines should be banned, I am against that. All such bans and forms of gun control do is give an advantage to the criminal and potential tyrant who would either disregard the law or would use the law to harm the law-abiding citizen. I had a long argument with a coworker yesterday about guns/gun control. This guy is a good fellow and very conservative about most things. However, when it comes down to the subject of guns, he can't understand why anyone other than law enforcement and the military should be allowed to own them. He makes allowances for me, saying that he doesn't mind me owning them, because, as he says, he knows me and that I am responsible, etc. After listening to him foam at the mouth for about 10 minutes, I ended the exchange by saying "if you want to want to be disarmed and helpless, so be it. However, if you think you are going to force me to be that way, you are sadly mistaken."
There were no drum mags found at the scene or used by this idiot. Every picture I’ve seen of the AR, every picture from the inside of the theater, all of the evidence points to the use of standard 20 round magazines.
My bet’s on garbage milsurp .223 as the reason for the jam.