In 2011, a shooting in Tucson, Arizona, was stopped when the gunman stopped firing in order to change magazines. The "high-capacity" magazine on his Glock pistol allowed him to fire 31 rounds before pausing. The Glock company website said that a standard magazine for its 9 mm semiautomatic pistol was between 13 and 17 rounds.
In the 2012 Aurora shooting, a 100-round drum magazine used by the shooter jammed. According to one senior FBI agent, "If his firing mechanism had not seized, he could have wiped out the entire audience in a few minutes." Although they are capable of delivering a large number of rounds, drum magazines are known to be unreliable, and heavy enough to significantly reduce accuracy.
The debate regarding magazine capacity intensified in the U.S. after the 2012 Sandy Hook Elementary School shooting where an AR-15 style rifle with 30-round magazines were used by the shooter. In January 2013, President Barack Obama included a ban on gun magazines with capacities of more than 10 rounds in a list of gun-control laws he asked Congress to act on, but no legislation was successfully passed.
The gunman in the 2013 Los Angeles International Airport shooting used a 30-round magazine for his Smith & Wesson M&P-15 rifle. That was one of 17 high-profile mass shootings which involved the use of "high-capacity" magazines that were cited by the Los Angeles City Council when it passes a ban on possession of such magazines in 2015. Other local incidents cited by supporters of the ban include the Los Angeles Jewish Community Center shooting and the 2013 Santa Monica College shooting. Syed Rizwan Farook and Tashfeen Malik used 30-round magazines illegal in California during the 2015 San Bernardino attack. Four such magazines were in their possession. Following the attack, there were calls for banning possession of "high-capacity" magazines in California.
https://en.wikipedia.org/wiki/High-capacity_magazine