This product allows you to store a spare AR mag behind other equipment (pistol mags) on your duty belt. It looks promising.
http://www.policeone.com/policeone/d...nterMassPR.jpg
It is said to accommodate 20 and 30 rnd USGI, PMAG or Lancer's.
http://www.centermassinc.com/Integrated_mag_pouch.html