Holmes’ little arsenal is worth attending to. Especially the 100-round magazine and the ammunition dump that would keep a Syrian rebel unit in business for days. It was all legally purchased by a guy whose lifetime police record seems to have comprised one (1) speeding ticket. We don’t even know what kind of stuff he had in his apartment because we’re still afraid to try to get in for a look-see.

There is no sporting purpose nor personal defense value in a 100-round magazine; I’m not sure it even has a milspec number. (It does figure on Thompson submachine guns, in gangster movies and probably in resolving real turf disputes and other misunderstandings among uomini di rispetto at least back in the day, maybe still.) Same with the 31-shot magazine Loughner got to empty at Giffords’ group last year. People who like to play with this stuff are quick to defend their rights to shred targets and watermelons down at the range for fun, but we regretfully prohibit even people whose pleasure therefrom might be enormous from messing with things like Ebola virus, or maintaining a stash of C4, or a flesh-eating staph zoo on the kitchen shelf. No-one ever killed anyone with a reefer, and heaven knows lots of folks enjoy using them for fun, but we don’t even allow people to own weed (whether that benefit-cost analysis is correct is another story; the principle is what matters here). With all due respect for privacy, we at least need a database system, based on solid identification, for purchases of stuff like this (I mean magazines far beyond any utility to a hunter, and thousands of rounds of ammunition) that allows authority to correlate it and do some serious investigation of the buyer.

Colorado is a concealed-carry state, but the movie theater didn’t allow customers to pack heat. What if it had? What happened is terrifying in itself, but if you really want to lie awake nights, imagine a dark, smoky, crowded theater filled with screaming people, Holmes firing away, plus a half-dozen or so armed citizens blazing away at “the guy with the gun”, meaning, assuredly, each other in addition to the perp with the big advantage of body armor, black clothes, and that 100-round AR-15. This kind of peacemaking takes place backstopped by people (including people in the next theater behind a wall), and even your best-trained vigilante’s aim is much impeded by smoke, darkness, noise, and being bumped into by the terrified guy next to you, not to mention being shot first by Holmes who might be especially hostile to someone pointing a gun at him. Bad as this was, not having amateurs mixing in spared us a much worse tragedy, in which Holmes might have taken a hit or two to the bulletproof vest before he walked out, but a lot more patrons would have left in body bags. (David Weigel has some more reflections on this debate, if you can call it that.)

[Cross-posted at The Reality-based Community]

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Michael O'Hare is a Professor of Public Policy at the University of California, Berkeley.