The Glock 21 was based on the Glock 20 but firing the large American .45 ACP cartridge.
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The Glock 21 (Glock GmbH of Austria) was essentially the Glock 20 model modified for the US market to fire the .45 ACP. The .45 ACP (or ".45 Auto") cartridge was a rimless cartridge developed by American gunsmith John Browning in 1904. The .45 ACP was featured in the legendary Colt M1911 semi-automatic pistol which became a staple of the US Army in 1911. The large cartridge proved effective enough though at the expense of its low-energy performance. Nevertheless, the Glock 21 existed to feed a market need for a police sidearm and general self-defense weapon.
The Glock 21 featured a larger frame than previous Glock iterations and appeared in production form in 1990. The Glock 20 frame was used as the starting point for the Glock 21, making room for a new magazine to take on the larger round as well as a new slide. The barrel featured an octagonal bore as opposed to the hexagonal types featured in other Glocks. The grip was enlarged to accept the new magazine which made the new Glock 21 comfortable in the grip of "large-handed" individuals. The standard magazine was of 13-rounds, this once again, due to the larger cartridges and these cartridges had to be staggered within the magazine. Like other semi-automatic pistols, the magazine was inserted into the base of the handgrip. Unloaded weight was 26.28 ounces. Length was 7.59 inches with a barrel length of 4.61 inches.
Externally, the Glock 21 looked very much like Glock 20 with the most notable exception being the enlarged grip.