A high-capacity magazine ban is a law which bans or otherwise restricts high-capacity magazines, detachable firearm magazines that can hold more than a certain number of rounds of ammunition. For example, in the United States, the now-expired Federal Assault Weapons Ban of 1994 included limits regarding magazines that could hold more than ten rounds. Eight U.S. states, and a number of local governments, ban or regulate magazines that they have legally defined as high-capacity. The majority of states (42) do not ban or regulate any magazines on the basis of capacity. States that do have large capacity magazine bans or restrictions typically do not apply to firearms with fixed magazines whose capacity would otherwise exceed the large capacity threshold.
The federal ban of 1994 defined a magazine capable of holding more than 10 rounds of ammunition as a large capacity ammunition feeding device. Likewise, the state of California defines a large capacity magazine as "any ammunition feeding device with a capacity to accept more than 10 rounds." Such devices are commonly called high-capacity magazines. Among states with bans, the maximum capacity is 10 to 20 rounds. Several municipalities, such as New York City, restrict magazine capacity to 5 rounds for rifles and shotguns. The state of New York previously limited magazine capacity to 7 rounds, but a District Court ruled this ban to be excessive and could not "survive intermediate scrutiny".
Most pistols sold in the U.S. are made and sold with magazines holding between 10 and 17 rounds. In November 2013, the National Rifle Association sued the city of San Francisco over an ordinance banning possession of magazines capable of holding more than 10 rounds. At the time, no court had overturned a ban on high-capacity guns or magazines. In March 2014, the Supreme Court refused to halt a similar ban by the city of Sunnyvale, California. Most Americans support a ban on the sale and possession of high capacity magazines.
William B. Ruger, a founder of Sturm, Ruger & Co., is often ascribed with providing the impetus for high capacity magazine restrictions. Ruger proposed that instead of banning firearms, Congress should outlaw magazines holding more than 15 rounds. On March 30, 1989, Ruger sent a letter to every member of the US Congress stating:
"The best way to address the firepower concern is therefore not to try to outlaw or license many millions of older and perfectly legitimate firearms (which would be a licensing effort of staggering proportions) but to prohibit the possession of high capacity magazines. By a simple, complete and unequivocal ban on large capacity magazines, all the difficulty of defining 'assault rifle' and 'semi-automatic rifles' is eliminated. The large capacity magazine itself, separate or attached to the firearm, becomes the prohibited item. A single amendment to Federal firearms laws could effectively implement these objectives."William B. Ruger
The Federal Assault Weapons Ban of 1993 included a ban on magazines capable of holding more than ten rounds of ammunition.:1–2 The Public Safety and Recreational Firearms Use Protection Act, commonly called the assault weapons ban (AWB), was enacted in September 1994. The ban, including its ban on magazines capable of holding more than ten rounds of ammunition, became defunct (expired) in September 2004 per a sunset provision. Attempts to renew the ban have failed on the federal level.
In Virginia, high-capacity magazines are not banned, but a semi-automatic, centerfire rifle or pistol with a magazine able to hold more than 20 rounds is considered an "assault firearm" and may only be purchased by citizens or permanent residents.[not in citation given (See discussion.)]
U.S. cities with high-capacity magazine restrictions or bans include:
In December 2013, the National Rifle Association (NRA), representing five residents of Sunnyvale, California, filed a lawsuit to stop enforcement of the city's ban on possession of magazines able to hold more than 10 rounds, in a case known as Fyock v. Sunnyvale. In March 2014, Supreme Court Justice Anthony Kennedy rejected a request to block enforcement of the law pending appeals. In March 2015 the federal 9th Circuit Court of Appeals upheld the magazine capacity restriction, ruling that it does not violate the Second Amendment.
Eugene Volokh, constitutional scholar, says that a Federal District Court judge was correct to decide that a local high-capacity magazine ban was constitutional, comparing limits on magazine capacity to limits on free speech.
Most Americans support a ban on the sale and possession of high capacity magazines. In 2012 62% of Americans favored banning the sale of high-capacity ammunition magazines, according to a Gallup poll with a margin of error of +/- 4%. In 2017 65% of American adults supported banning high-capacity magazines, according to a Pew Research Center survey with an error attributable to sampling of +/- 2.8% at the 95% level of confidence. In late February early March 2018, after the Stoneman Douglas High School shooting, 63% of American adults supported a ban on the sale and possession of high-capacity or extended ammunition magazines, according to a CNN poll with a margin of error of +/- 3.7%. 73% of American adults supported banning high-capacity ammunition magazines that hold more than 10 rounds, according to an NPR/Ipsos poll with a margin of error of +/- 3.5%. 70% of registered voters supported banning high-capacity magazines, according to a Politico/Morning Consult poll with a margin of error +/- 2%.
With the passage of Bill C-17 in 1993 under Prime Minister Kim Campbell (in response to the 1989 Ecole Polytechnique Massacre), magazines designed for use in semi-automatic centrefire rifles and semi-automatic shotguns became limited to five rounds, and magazines designed for use in handguns are limited to 10 rounds. Magazines designed for use in semi-automatic rimfire rifles, as well as manually operated long guns, are exempt from the magazine capacity restrictions.
In recent years, there has been a growing trend of ways to legally work around the magazine capacity restrictions. Numerous semi-automatic centrefire rifles also happen to accept handgun magazines, thereby legally increasing magazine capacity. Numerous rifle and handgun magazines designed for a particular caliber also happen to fit an over-the-limit number of smaller caliber rounds, also legally increasing magazine capacity. 
The ban on assault weapons and high-capacity magazines has been a crucial component of our national crime-fighting strategy.
In the context of this book, however, 'assault weapon' refers to a semi-automatic firearm that accepts high capacity magazines (10+ rounds) and is patterned after military issue select-fire weapons.
in fact, most pistols sold in the U.S. are made and sold with magazines holding between 10 and 17 rounds.
Magazines that carry more than 10 rounds are widely sold in the United States and can't be considered 'dangerous and unusual,' Tuesdays lawsuit asserted.
There is widespread support for several specific changes to gun laws, including 87% who back laws to prevent convicted felons and those with mental health problems from owning guns; 71% who support preventing people under age 21 from buying any type of gun; 63% who support a ban on the sale and possession of high-capacity or extended ammunition magazines (up from 54% in October, a new high in CNN polling); and 57% who back a ban on the manufacture, sale and possession of rifles capable of semi-automatic fire, such as the AR-15, the same style as was used in both the Parkland and Las Vegas shootings (up from 49% in October).
Some specific measures that are broadly popular: 87 percent favor banning felons and those with mental health problems from getting guns; 71 percent favor banning anyone under 21 from buying a firearm; and 63 percent favor a ban on high-capacity magazines.
Seventy percent of voters support a ban on high-capacity magazines, and 68 percent want to ban assault-style weapons.