The MEAN ExoMag is designed to be the most reliable and best feeding 9mm magazine for use with a standard AR lower. The ExoMag has a STANAG magazine outer shape. Add the 9mm upper of your choosing to function with a Glock or Colt variant 9mm BCG. Mean offers both a version with an EJECTOR (for standard blowback bolts) and an EJECTORLESS version (for our Bearing Delay and CMMG Radial Delay systems). All ExoMag models include the last round bolt-hold open feature, as any AR-15 should. Bearing Delay Upper owners: Customers with the Bearing Delay system should select the "Ejectorless" option in the "ExoMag Package Selection" option above. CMMG Mfg: Mean Arms