Magazines holding more than 10 rounds have been common in America for over 150 years.

From Dave Kopel of the Volokh Conspiracy via Instapundit:

Currently before the Ninth Circuit is an appeal in the case of Fyock v. Sunnyvale, a case which challenges a California’s city’s ban on magazines which hold more than 10 rounds. While the State of California outlaws the sale, import, or transfer of such magazines, the Sunnyvale ban goes further, by prohibiting possession of these magazines, with no provision for grandfathering. The District Court upheld the ban; part of the Court’s analysis stated that magazines did not exist at the time the Second Amendment was ratified. Last Friday, amicus briefs in support of appellant were filed, including a brief which I co-authored on the history of magazines and of magazine prohibition.
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The longest section of brief provides the history of magazines, with a particular focus on magazines holding more than 10 rounds. The first such magazine was invented in the late 16th century. Over the next two centuries, many inventors created guns with magazines holding more than 10 rounds. The consumer demand for such magazines is easy to understand: when a gun is out of ammunition, the user is in effect disarmed, and does not possess an operable firearm until the gun is reloaded. As Heller teaches, the Second Amendment protects the right to an operable firearm.

Whoops, that is going to put a whole in the District Court's thinking.