The AR is the text book definition of "common use". It's the most popular model firearm in the country. If that doesn't define common - then everything could be banned.
Your post made me look up how many ARs are in the US today. No definite numbers but found this article.
http://www.usatoday.com/story/news/...ry-style-ar-rifles-market-saturated/19836755/
In 2012, Slate.com crunched numbers from a variety of manufacturers, as well as federal statistics on background checks, and extrapolated that nearly 3.3 million AR-15s were in the country. But that was before Newtown and calls for bans, which drove sales through the roof.
In congressional testimony last year, the National Shooting Sports Foundationestimated assault-style weapons domestically in the range of 5 million to 8.2 million.
Even 8 million seems low to me. Everyone I know who owns a gun owns at least one ar. And many own multiple. One guy I know owns over 30. (He wants extra to pass out during the zombie invasion)