This is a factory 13-round magazine for Gen 4 Glock 21 and 21SF pistols in .45 ACP Glock’s .45 ACP combat pistols are known for their massive capacity, easy-to-handle recoil, and ultra-reliable performance. Designed to feed consistently in even the most extreme environments, Glock factory magazines are the ONLY choice for serious shooters. These standard capacity, full-size factory magazines give you 13 rounds of big bore .45 ACP in a crack-resistant polymer magazine with full length reinforcing steel inserts, steel spring wire internals, and numbered witness holes.