A high-capacity magazine (or large-capacity magazine) is a firearm magazine capable of holding more than the "normal" amount of ammunition, some number of cartridges dependent on jurisdiction and kind of firearm. For some legal purposes in some jurisdictions, magazines holding more than 10 rounds are considered "high-capacity".
In the U.S., since the 1980s, magazine capacity has been a subject of debate regarding civilian firearms. Many assault weapon bans since then have included or been accompanied by large-capacity or high-capacity magazine bans. Magazine capacity is also debated among military users, who have to balance the greater firepower of high-capacity magazines with their greater weight and size.
As a legal and commercial term, it has various definitions. The National Rifle Association (NRA) defines high-capacity magazine as "[an] inexact, non-technical term indicating a magazine holding more rounds than might be considered 'average.'"
High-capacity magazine Wikipedia
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 website said that a standard magazine for its 9mm semiautomatic pistol was between 13 and 17 rounds. A 100-round magazine used in the 2012 Aurora shooting 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."
The debate regarding magazine capacity intensified in the U.S. after December 2012 Sandy Hook Elementary School shooting. 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 pass. 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.
Gun rights groups such as the Jews for the Preservation of Firearms Ownership (JPFO) have endorsed the use of "high-capacity" magazines.