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.