Glock handguns are available in seven different calibers among 21 different models. The basic configurations are full-size models with 4.5-inch barrels, the slightly smaller compact models with 4-inch barrels, the subcompact models designed for concealed carry with 3.5-inch barrels and the competition models that are full-size pistols with longer 5.3-inch barrels.
The original model of Glock pistols was the G17 that featured a 17-round capacity and was cambered for 9mm ammunition. Glock has four 9mm models: the original full-size G17, the compact model G19, the subcompact G26 and the G34 competition model. The 9mm bullet is the standard pistol round used by the American military and most NATO forces.
The .380 bullet is essentially a 9mm bullet with a slightly shorter shell casing and is also, but less commonly, referred to as a 9mm short. Glock begin producing .380 handguns specifically for South American shooters who are restricted to only using weapons of nonmilitary calibers. The Glock .380 handguns are the compact G25 and the subcompact G28.
The .40 S&W (so named because the bullet was developed by Smith & Wesson) is a very popular bullet among law enforcement and civilian shooters alike. The bullet is seen as a compromise between the small 9mm and the large .45; the size of the bullet allows for a greater magazine capacity, while the ballistics are powerful enough for law enforcement applications. Glock produces the full-size G22, the compact G23, the subcompact G27 and the G35 competition model.
The .357 SIG (developed by SIG Sauer) is a high-velocity bullet that uses the same shell casing has a .40 S&W. Glock produces the full-size G31, the compact G32 and the subcompact G33. The barrels from comparable .40 S&W models are interchangeable with .357 SIG models, allowing you to shoot .40 S&W bullets from your .357 SIG pistol by switching the barrels. The same is true for shooting .357 SIG bullets from your .40 S&W pistol.
The 10mm bullet is a high-velocity round used primarily for hunting. Glock only produces two 10mm models: the full-size G20 and the subcompact G29.
The .45 ACP (Automatic Colt Pistol) is the largest caliber offered in a Glock handgun and is available in the full-size G21, the subcompact G30 or the Slimline G36. The G36 is a subcompact pistol with a smaller magazine capacity to make the weapon easier to conceal.
The .45 GAP (Glock Automatic Pistol) is a shortened version of the .45 ACP but with the same performance. The bullet was developed by Glock and is available in the full-size G37, the compact G38 and the subcompact G39.