In the first place, a halfway competent shooter can quickly replace an empty magazine with a fresh one, or else switch to another gun. (Holmes allegedly used three and had a fourth.)
The brief interruption a killer needs for reloading is helpful only if someone can seize the moment to subdue him -- something more common in movies than in real life. Florida State University criminologist Gary Kleck says he knows of only one mass shooting in which that happened, in 1993. In the 2011 Tucson shooting, the suspect was overcome when his gun jammed after he reloaded.
Tracking anyone who makes large ammunition purchases? David Kopel, research director at the free-market Independence Institute in Denver, points out that more than a billion rounds are sold each year in the United States -- many of them in bulk by target shooters who burn through hundreds or thousands every month.
If the Bureau of Alcohol, Tobacco, Firearms and Explosives were to investigate each of these buyers, it would have little time to do anything else. And it would probably catch no criminals, since they would buy in smaller lots to avoid detection.
Besides, most of the rounds that Holmes allegedly bought lay idle. The quantity of ammunition he is said to have used could have been obtained in a few purchases that would set off no alarms. The rest of his fearsome stockpile had no bearing on the outcome.
Ideas like these are proposed anytime a mass shooting takes place but lately, at least, never go anywhere. Supporters take that as proof of the vast, unhealthy influence of the National Rifle Association. But it could be Americans just have no appetite for solutions that don't solve.