Two, belt-mounted, AR-sized magazine carriers that I like are the ITW Fastmag and the Blade-Tec Revolution.
ITW Fastmag without tabs for belt mount (tabs for use with Molle):
Revolution AR-15/M4 Single Mag Pouch
The BladeTec uses a Tek-Lok, so it's pretty easy to add/remove from a belt. The Fastmag uses a different belt attachment, but you can stack the singles to make double/triple/etc carriers.