MagPump Magazine Retainer 9mm Double Stack Magazines STI Black MR-STI. In an effort to pump up the performance level of one of America's most popular handgun calibers MagPump introduces new double stack magazine retainers The retainers come in a set of six and are designed to fit popular double stack magazines. The pump is easily installed by hand and features an ergonomically designed barricade stop on the lower face. The pump's end face now features a ribbed surface to prevent the magazine from sliding out of the way. This retainer is the same excellent quality as the original that comes with the MagPump magazine loaders but in a double stack format. The material is still the same and features the patented 'ribs' which give the magazine a positive locking feature. However the MagPump retainer fits much more than just double stack magazines. The retainers have been designed by MagPump to handle the increased magazine capacity of the double stack magazines. The pump is easily installed by hand with the aid of the Pump's speed loader support handle. The Pump magazine loader is a registered trademark of MagPump and is used with the explicit written consent of the Pistol Calibers for use with 9mm pistols. Specifications for MagPump STI 2011 9mm Double Stack Magazine Retainer: Manufacturer: MagPump Caliber: 9mm Luger Gun Make: STI International Gun Model: STI International 2011 Country of Origin: USA Package Contents: MagPump STI 2011 9mm Double Stack Magazine Retainer MagPump MR-STI 2011 : Magazine Retainer 9mm Double Stack Magazines STI 2011 Black