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 but was shorter to fit in a more compact handgun, and with a stronger case head to reduce the possibility of case neck blowouts. G.A.P. is an acronym for "Glock Automatic Pistol", and the .45 G.A.P. is the first commercially-introduced cartridge identified with Glock
The concept of a shorter .45 ACP round is not new; previously there was the .45 H.P., no longer in production. However, the .45 GAP had the backing of a major firearms manufacturer, which greatly increased its chances for success given Glock pistols' popularity and market share, particularly among law enforcement personnel.
The .45 GAP is the same diameter as the .45 ACP pistol cartridge, but is shorter than the .45 ACP and uses a small-pistol primer, instead of the large-pistol primer as found in most commercial loadings of the .45 ACP. Originally, the maximum weight of the .45 GAP was 200-grain (13 g), and in order to provide terminal ballistics on par with the standard 230-grain (15 g) .45 ACP "factory loads" the .45 GAP was designed to operate at a higher standard pressure – roughly equivalent to the higher pressure found in the .45 ACP "+P" rounds. The designer was only able to achieve the desired pressure and resulting velocity through clever powder selection, since the .45 GAP has a much smaller cartridge volume than the .45 ACP. Later development concluded that the .45 GAP could in fact fire 230-grain (15 g) ammunition just as the .45 ACP.
The full-frame sized Glock 37 pistol (at full size the Glock 37 is still considered a medium-framed pistol) was introduced by Glock to use the .45 GAP and was followed by both the compact (Glock 38) and the sub compact (Glock 39) models. Width of both pistols is listed by the manufacturer as 1.18" compared with 1.27" for the sub compact Glock 30 (.45 ACP), indicating Glock was not only able to shorten the front to back dimensions, but also the width of the grip. The GAP models holds 10, 8, and 6 rounds respectively, however, compared to 13 and 10 in the Glock 21 and 30, respectively. The introduction of "Short Frame" .45 ACP versions of the "full-sized" Glock 21 and the "sub compact" Glock 30 have lessened the need for the .45 GAP in terms of satisfying the requirements of law enforcement agencies which must offer sizes for smaller hands, the 45 GAP Glocks still remain thinner (comparable to the "single stack" Glock 36 in .45 ACP) and does not require reduction of the back strap like the Glock 21 "Short Frame" and has a much shorter grip length then the Glock 21 .45 ACP and therefore more attractive for some shooters. The question remains, however, in light of the introduction of thinner gripped .45 ACP's, such as the Smith & Wesson M&P .45, and shorter (front to back) gripped .45 ACP's, such as the Springfield Armory XD .45 (the back strap of the grip was reduced to shorten the grip at the cost of lower recoil absorption), whether the market niche for the .45 GAP will remain large enough to support it is unknown but many law enforcement agencies in the U.S. have adopted it as their standard issue duty weapon.
The smaller dimensions of the .45 GAP make it cheaper to produce but it is still a relatively new cartridge and is currently difficult to find in mass-marketing outlets and at many gun stores, it is available on-line from many sources and, when discounted, is available for an approximately 10% premium over "factory loaded" .45 ACP. When compared to "self defense" .45 ACP rounds, the price is very affordable.
Other similar rounds to consider are the .40 S&W and the 10 mm Auto but these calibers have been said to have a "snappier" recoil when compared to the .45 ACP. Although 9 mm is available in high-performance rounds, the .40 S&W and the 10 mm are almost always loaded "hot," while the .45 ACP has been considered effective even in "lighter" loads due to its larger (than 9 mm or .38 special) diameter and heavier bullet weight.
Gun Tests Magazine ran an article in the February 2005 issue comparing five commercial loadings of .45 GAP ammunition when fired through the Glock 37 polymer-framed pistol. All of the brands failed to meet the desired 2-inch (51 mm) accuracy at 50 feet (15 m); the groups ranged from 2.5 to 3.5 inches (64 to 89 mm). 4 of the 5 loads generated velocities on par with the .45 ACP +P, and these loads were considered uncontrollable by the testers, as the recoil generated was extreme for the weight of the pistols chambering the round. The one remaining load, firing a 185-grain (12 g) bullet at an average velocity of about 950 ft/s(290 m/s), was considered to be at the upper limit of controllability. The reviewer theorized that the potency of the .45 GAP factory loads was an attempt to avoid derogatory comparisons between the .45 ACP and the .45 GAP, such as was experienced when the .40 S&W was introduced as a lower recoil substitute for the 10 mm Auto.