In 1994, Congress passed a 10-year ban on the manufacture and sale of new assault weapons, which the law defined as semi-automatic rifles and handguns with certain military-style features -- such as folding rifle stocks and threaded barrels for attaching silencers -- that didn't have any value to hunters or self-defense. The law also banned magazines with a capacity of more than 10 rounds but exempted weapons manufactured before 1994. The law was allowed to expire in 2004, and how effective it was at preventing crime remains a subject of intense controversy, in part because there wasn't a systematic effort to gather data about its impacts.
A 2004 study by University of Pennsylvania researchers for the Department of Justice found that from 1995 to 2003, gun crimes involving assault weapons that were banned by the law declined in six U.S. cities by between 17 percent and 72 percent. But some of that progress was negated, the researchers found, because even though criminals couldn't buy new assault weapons, they still could easily outfit non-banned weapons with old large-capacity magazines from before the ban, which were plentiful and easily obtained [source: Koper].
Additionally, manufacturers were able to get around the ban by redesigning weapons and making a few changes to remove the military-style features [source: Peterson]. The Colt AR-15 that suspect James Eagan Holmes used to kill moviegoers in the Aurora cinema would have been outlawed under the 1994 ban. Yet he could have used a very similar Colt Match Target rifle that would not have fallen under the ban [source: Plumer].