Glock G21 .45 ACP 13 Round Pistol Magazine Black 1 Pack 13. Factory Glock magazines have a hardened steel insert encased in high tech polymer. This polymer coating protects the magazine and prevents deformation even when dropped from a great height. Note: Glock magazines can be very difficult to load when new. Often the last round will not fit until the magazine has undergone a break-in period. It is recommended that the magazine be loaded with all but the last round and left for a few days to compress the spring. Cycling the magazine from full to empty multiple times can assist in the break-in period. Specifications for Glock G21 .45 ACP 13 Round Pistol Magazine: Manufacturer: Glock Gun Make: Glock Gun Model: Glock 21 Caliber: .45 ACP Magazine Capacity: 13 Round Color: Black Fabric/Material: Polymer Package Contents: Glock G21 .45 ACP 13 Round Pistol Magazine What firearm models does the manufacturer say Glock G21 .45 ACP 13 Round Pistol Magazine is compatible with? The manufacturer suggests that Glock G21 .45 ACP 13 Round Pistol Magazine is compatible with Glock 21Glock 1098: G21 .45 ACP 13 Round Pistol Magazine Black 1 Pack 13