The .45 G.A.P. (often called the .45 "GAP") pistol cartridge was designed by Ernest Durham, an engineer with CCI/Speer, at the request of firearms manufacturer Glock to provide a cartridge that would equal the power of the .45 ACP, have a stronger case head to reduce the possibility of case neck blowouts, and shorter to fit in a more compact handgun. G.A.P. is an initialism for "Glock Automatic Pistol", and the .45 G.A.P. is the first commercially introduced cartridge identified with Glock.
.45 GAP Wikipedia
The .45 GAP has the same diameter as the .45 ACP pistol cartridge but is slightly shorter, and uses a small-pistol primer instead of the large-pistol primer most commonly used in .45 ACP ammunition. Originally, the maximum bullet weight of the .45 GAP was 200 grains (13 g). In order to provide terminal ballistics that matched the standard 230-grain (15 g) .45 ACP loads, the .45 GAP was designed to operate at a higher standard pressure—roughly equivalent to the higher pressures found in .45 ACP "+P" rounds. Since the .45 GAP has a much smaller cartridge volume than the .45 ACP, the desired pressure and resulting velocity needed to be achieved through powder selection alone. Later development concluded that the .45 GAP could also fire 230-grain (15 g) projectiles as does the .45 ACP.
The full-size Glock 37 pistol was introduced by Glock to use the .45 G.A.P. cartridge and was followed by the compact Glock 38 and the subcompact Glock 39. Glock's .45 GAP sized pistols use the same frame as their 9×19mm/.40 S&W/.357 SIG line of pistols. The slide is slightly wider to accommodate the larger diameter .45 cal round and is flush with the frame. Magazines for the .45 G.A.P. are of the same dimensions as those that the 9×19mm/.40 S&W/.357 SIG line of pistols use.
Four state law enforcement agencies have adopted the .45 G.A.P. as a replacement to their current issue 9×19mm Parabellum (New York) or .40 S&W service handguns ( South Carolina, and Florida). The New York State Police, South Carolina Highway Patrol, and Florida Highway Patrol have all adopted the Glock 37 and .45 G.A.P.
Initially, due to its acceptance by law enforcement and the popularity of subcompact handguns for concealed carry, a small number of manufacturers decided to produce pistols that were chambered in .45 G.A.P., but they no longer produce any pistols in that caliber. Only Glock and Bond Arms continues to manufacture pistols in the 45 G.A.P. cartridge.