To Summarize the Main Reasons for the New .45 GAP Caliber
To propel the same .45 bullets as the .45 ACP at the same general velocities and with the same inherent accuracy as the .45 ACP.
To use a shorter, more efficient .45 cartridge that functions reliably in 9mm/40S&W subcompact, compact, and full size pistols with capacities ranging from 6 up to 12 rounds, and with a grip size that works well with any hand size.
To take advantage of the new 45 GAP cartridge that was engineered to be safely fired in the typical unsupported barrel chamber. The stronger GAP case has more of a safety buffer than both the standard .45 Auto and .40 S&W in order to protect against out of spec ammo and/or pistols.
To allow other manufacturers to quickly convert their 9mm, .40, & .45 based auto pistols into .45 GAP pistols with low R&D costs. How is this possible? Because the shorter .45 GAP cartridge allows the use of 9mm/.40 sized pistol frames and the Glock magazine is semi-double stacked to accommodate 10 rounds of the fat bullet. On the other hand, the slightly longer .45 ACP cartridge requires a larger pistol frame and larger magazine and grip to accommodate the longer cartridge. Therefore 9mm/.40 sized pistols can easily be retooled to support the .45 GAP while this is not possible with the venerable .45 ACP. For example: a .45 GAP would most likely work in a modified Kahr, Sig239, Sigpro, etc, while a .45 ACP would not. It has already been officially accomplished with the Springfield XD (4” & 5”), and Glock 37/38/39.
To take advantage of the .45 GAP costing less than the .45 ACP to produce and load: less powder to do the same thing as the acp, small pistol primers, and a 1/7th inch shorter case saves on unneeded case metal, which even provides less overall weight than the .45 ACP cartridge in bulk.
To demonstrate that .45 GAP and .45 ACP pistols of equal size and ammo of equal power provide a similar general felt recoil since the GAP does the same thing as the ACP by just using less powder with a slightly higher pressure, although this pressure is still 34% less than 9mm/.40 S&W calibers. “Full Power” 45GAP loads are 55 to 70 fps slower than “Full Power +P” 45ACP loads (although Double Tap GAP ammo is equivalent to +P 45ACP ammo) when using the same barrel lengths. This essentially means that the hottest 45GAP loads are a little faster than standard pressure 45ACP loads and therefore 45GAP ammo can be downloaded to easily match standard pressure 45ACP loads for easy controllability.
To take advantage of the short .45 GAP cartridge that can go through the semi-auto feed, extract, and ejection cycle faster than a long cased ACP cartridge. The GAP may work better in micro and subcompact models because of the short slide cycle time which can make it trickier for the longer cased .45ACP.
To take advantage of the slightly higher pressure in the 45 GAP compared to the 45 ACP.
To take advantage of using the short .45 GAP cartridge to load and unload revolvers faster than is possible with the long .45 ACP cartridge.
In a nutshell, the 45 GAP cartridge is an excellent fit in performance, pistol size, cartridge size, and reinforced case strength for added safety that is beneficial to all shooters. And don't forget the magnum power without the noise, 11.5mm/.451 bullet compared to 10mm/.40 & 9mm bullets, as well as using significantly lower pressures than most duty calibers.
For those that really need that full power buzz and bragging rights to having the fastest chariot in town, then by all means get an ACP pistol so you can shoot +P ammo (60 – 100 fps faster than GAP factory ammo, except for Double Tap GAP ammo which is just as fast as +P 45ACP ammo) and so you can shoot .45 Super ammo and get a sore hand to boot. It's more important to use a correctly tuned bullet for a given velocity.