The .45 ACP is an effective combat pistol cartridge that combines accuracy and stopping power for use against human targets. It has relatively low muzzle blast and flash, and it produces moderate recoil in handguns, made worse in compact models. The standard issue military .45 ACP round has a 230-grain bullet that travels at approximately 830 feet per second when fired from the government issue M1911A1 pistol and approximately 950 feet per second from the M1A1 Thompson submachine gun. The cartridge also comes in various specialty rounds of varying weights and performance levels.
It operates at a relatively low maximum chamber pressure rating of 21,000 psi (145 MPa) (compared to 35,000 psi/241 MPa for 9mm Parabellum and .40 S&W, 37,500 psi/259 MPa for 10mm Auto, 40,000 psi/276 MPa for .357 SIG), which due to a low bolt thrust helps extend service life of weapons in which it is used. Some makers of pistols chambered in .45 ACP do not certify them to use Plus P ammunition.
In its non-expanding full metal jacket (FMJ) version, the .45 ACP cartridge has a reputation for effectiveness against human targets because its heavy mass has the capacity to penetrate tissue deeply and damage the central nervous system, and its large 11.5mm diameter creates a more substantial permanent wound channel than other calibers, which can lower blood pressure rapidly if critical organs of the circulatory system are hit.
Drawbacks for military use include the cartridge’s large size, weight, increased material costs in comparison to the smaller, flatter shooting NATO standard 9×19mm Parabellum cartridge, which uses less powder, brass, and lead per round. Standard 9mm NATO ammunition has limited armor penetration capability − a deficiency with .45 ACP whose large, slow bullet does not penetrate armor to any great extent. The low muzzle velocity also makes the bullet drop over long ranges, making hits more difficult; however, it is important to note that the vast majority of self-defense situations involving handguns typically occur at close ranges.
Recent testing of the three major police and military calibers by the Federal Bureau of Investigation found that the .45 ACP was no more effective with regard to terminal ballistics than either 9 x 19mm Parabellum or .40 S&W. After two years of testing, one of the final FBI comments was that services that adopt (or stay with .40 S&W or .45 ACP) did so at the risk of increased recoil and a possible reduction in accuracy as 9 x 19mm with premium quality ammunition had nearly exactly the same performance. A factor rated by the recent FBI testing was accuracy and time to recover. The .45 ACP handguns ranked last, largely due to increased recoil. Additionally, some firearms selected were also less safe.
Because of its large diameter and straight-walled design, the .45 ACP geometry is the highest power-per-pressure production, repeating round in existence. This is because of the higher powers achievable with .45 Super, and +P loads. Because of these inherent low pressures of the standard pressure round, however, compensators and brakes have little effect until +P and Super loads are utilized.
45ACP – Wikipedia