|Place of origin||United States|
|Wars||World War I – present|
|Variants||.45 ACP +P, .45 Auto Rim, .45 Super, .460 Rowland|
|Case type||Rimless, straight|
|Bullet diameter||.452 in (11.5 mm)|
|Neck diameter||.473 in (12.0 mm)|
|Base diameter||.476 in (12.1 mm)|
|Rim diameter||.480 in (12.2 mm)|
|Rim thickness||.049 in (1.2 mm)|
|Case length||.898 in (22.8 mm)|
|Overall length||1.275 in (32.4 mm)|
|Case capacity||26.7 gr H2O (1.73 cm3)|
|Rifling twist||1 in 16 in (406 mm)|
|Primer type||Large (some makers are now using Small) Pistol|
|Maximum pressure (CIP)||19,900 psi (137 MPa)|
|Maximum pressure (SAAMI)||21,000 psi (140 MPa)|
The 45 ACP (Automatic Colt Pistol) or 45 Auto (11.43×23mm) is a rimless straight-walled handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it was adopted as the standard chambering for Colt's M1911 pistol. The round was developed due to a lack of stopping power in Sulu experienced in the Moro Rebellion using the .38 Long Colt. This experience and the Thompson–LaGarde Tests of 1904 led the Army and the Cavalry to decide a minimum of 45 caliber was required in a new handgun.
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. It operates at a relatively low maximum chamber pressure rating of 21,000 psi (145 MPa), compared to 35,000 psi (241 MPa) for both 9mm Parabellum and 40 S&W, which due to a low bolt thrust helps extend service life of weapons. Due to standard pressure 45 ACP rounds being inherently subsonic when fired from handguns and submachine guns, it is a useful caliber for suppressed weapons to eliminate the sonic boom.
Today, most NATO militaries use sidearms chambered for the 9×19mm Parabellum cartridge, but the effectiveness of the 45 ACP cartridge has ensured its continued popularity with large-caliber sport shooters, especially in the United States. In 1985, the 45 ACP M1911A1 pistol was replaced by the Beretta M9 9mm pistol as the main sidearm of the U.S. military, which in turn was replaced with the SIG Sauer P320 9mm pistol in 2017, designated M17 for the full-size and M18 for the compact.
During the late 19th century and early 20th century, the U.S. Cavalry began trials to replace their sidearm arsenal of issued 45 Colt Single Action Army (SAA) in favor of the more modern and versatile double-action revolver in 45 Colt.
After the example of the Cavalry, the Army in turn had fielded versions of double-action revolvers in .38 Long Colt. It was eventually evaluated that the .38-caliber round was significantly less effective in overall stopping-power than the 45 Colt against determined opponents in cases such as the Tausug Moro juramentado warriors, who were encountered in the Moro Rebellion. The then-current issue rifle, the .30-40 Krag, had also failed to stop Moro warriors effectively; the British had similar lack-of-stopping-power issues switching to the .303 British, which resulted in the development of the dum-dum bullet in an attempt to compensate for the round's deficiencies. This experience, and the Thompson–LaGarde Tests of 1904, led the Army and the Cavalry to decide a minimum of 45 caliber was required in a new handgun. Thompson and Major Louis Anatole La Garde of the Medical Corps arranged tests on cadavers and animal remains in the Chicago stockyards, resulting in the finding that 45 was the most effective pistol cartridge. They noted, however, training was critical to make sure a soldier could score a hit in a vulnerable part of the body.
Colt had been working with Browning on a .41 caliber cartridge in 1904, and in 1905, when the Cavalry asked for a 45 caliber equivalent, Colt modified the pistol design to fire an enlarged version of the prototype .41 round. The result from Colt was the Model 1905 and the new 45 ACP cartridge. The original round that passed the testing fired a 200 grain (13 g) bullet at 900 ft/s (275 m/s), but after a number of rounds of revisions between Winchester Repeating Arms, Frankford Arsenal, and Union Metallic Cartridge, it ended up using a 230 grain (14.9 g) bullet fired at a nominal velocity of 850 ft/s (260 m/s). The resulting 45-caliber cartridge, named the 45 ACP, was similar in performance to the 45 Schofield cartridge and only slightly less powerful while significantly shorter than the 45 Colt cartridge that the United States Cavalry was using at the time.
By 1906, bids from six makers were submitted, among them Browning's design, submitted by Colt. Only DWM, Savage, and Colt made the first cut. DWM, which submitted two Parabellums chambered in 45 ACP, withdrew from testing after the first round of tests, for unspecified reasons.
In the second round of evaluations in 1910, the Colt design passed the extensive testing with no failures, while the Savage design suffered 37 stoppages or parts failures. The Colt pistol was adopted as the Model 1911.
The cartridge/pistol combination was quite successful but not satisfactory for U.S. military purposes. Over time, a series of improved designs were offered, culminating in the adoption in 1911 of the "Cal. 45 Automatic Pistol Ball Cartridge, Model of 1911", a 1.273 in (32.3 mm) long round with a bullet weight of 230 grains (15 g). The first production, at Frankford Arsenal, was marked "F A 8 11", for the August 1911 date.
Other US military cartridges include: tracer M26 (red tip), blank M1921 (rolled crimp, red paper wad), M12 and M15 shot shells, and M9 dummy (holes in case).
The cartridge was designed by John Browning for Colt, but the most influential person in selecting the cartridge was Army Ordnance member Gen. John T. Thompson. After the poor performance of the Army's .38 Long Colt pistols evidenced during the Philippine–American War (1899–1902), Thompson insisted on a more capable pistol cartridge.
45 ACP maximum C.I.P. cartridge dimensions. All sizes in millimeters (mm).
SAAMI specifications for 45 acp. All dimensions are in inches (millimeter)
The common rifling twist rate for this cartridge is 1 in 16 in (406 mm), 6 grooves, Ø lands = .442" (11.23 mm), Ø grooves = 45" (11.43 mm), land width = .147" (3.73 mm) and the primer type is large pistol. The cartridge headspaces on the mouth of the case at the L3 datum reference.
According to Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives rulings, the 45 ACP cartridge case can handle up to 131 MPa (19,000 psi) Pmax piezo pressure. In CIP-regulated countries every pistol cartridge combination has to be proofed at 130% of this maximum CIP pressure to certify for sale to consumers. This means that 45 ACP chambered arms in C.I.P. regulated countries are currently (2016) proof tested at 170 MPa (25,000 psi) PE piezo pressure.
The 45 ACP is an effective combat pistol cartridge. It combines accuracy as well as stopping power for use against human targets, has relatively low muzzle blast and flash, and it produces a stout, but manageable recoil in handguns (made worse in compact models).
The standard issue, military 45 ACP cartridge contains 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 fired from the Thompson M1A1 submachine gun. The cartridge comes in various specialty rounds of varying weights and performance levels as well.
The cartridge 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 of its heavy mass, having the capacity to penetrate tissue deeply, and damage the central nervous system. Its large 11.5mm diameter creates a more substantial permanent wound channel versus smaller calibers, which can lower blood pressure rapidly if critical organs of the circulatory system are hit.
In its expanding hollow point form, it is also particularly effective against human targets. In tests against ballistic gelatin, a 185 grain hollow point traveling at 1,050 feet per second expanded to about .76 inches. This is a significantly large permanent wound cavity for a handgun projectile. For those who follow the energy dump and/or hydrostatic shock theories of wounding ballistics, this is ideal. While slightly decreasing penetration and likewise the chance of hitting a vital organ, a large diameter wound will cause more blood loss. There is also a reduced likelihood of overpenetration, meaning that it is more likely that the projectile will transfer all of its kinetic energy to the intended target, thus more reliably incapacitating them.
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, a cartridge which uses less powder, brass, and lead per round. Standard 9mm NATO ammunition has a more limited armor penetration capability − a deficiency shared 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.
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.
Because of its large diameter and straight-walled design, the 45 ACP geometry is the highest power-per-pressure repeating production round in existence. This is because of the higher powers achievable with 45 ACP +P, 45 Super, and 460 Rowland loads. Because of the inherent low pressure of the standard round, however, compensators and brakes have little effect until +P, Super, and Rowland loads are utilized.
As standard pressure 45 ACP rounds fired from handguns and submachine guns are inherently subsonic, it is one of the most powerful pistol calibers available for use in suppressed weapons since subsonic rounds are quieter than supersonic rounds. The latter inevitably produce a highly compressed shock wave, audible as a loud "crack", a small sonic boom, while they travel through the air. Suppressors reduce the audible "report" by slowing and channeling the high speed gas generated by the burning/expanding gunpowder before it exits the muzzle resulting in a muffled "cough". Suppressors cannot act on a supersonic shock wave continuously generated by a bullet exceeding the 1,087 ft/s (331 m/s) speed of sound at 32 °F (0 °C) ambient cold temperatures, as this shock wave is continuously produced throughout the entire flight path over which the bullet is supersonic, which extends long after it exits the barrel.
The downside to the use of 45 ACP in suppressed weapons is that increasing the diameter of the passage through a suppressor decreases the suppressor's efficiency; thus, while 45 ACP is among the most powerful suppressed pistol rounds, it is also one of the loudest. Most 45 suppressors must be fired "wet" (with an ablative medium, usually oil or water) to bring sound levels down to "hearing-safe" (under 140 dB, generally).
Magazine capacity varies depending on the type of firearm. Standard (not extended) single-stack magazines, pistols based on the 1911 design commonly hold 8 rounds or less. Many modern pistols have adopted the cartridge into double-stacked magazine designs to increase ammo capacity, though this increases the pistol's handle width. Drum magazines used mostly for machine guns have a capacity of 50 rounds.
Several US tactical police units still use the 45 pistol round. While high capacity firearms are available in 45 ACP, the greater length and diameter of the 45 ACP means that the grip of the pistol must be longer and wider than the grip of a comparable pistol of a smaller caliber; this increase in grip size can make the pistol difficult to use for shooters with smaller hands.
Today, most NATO militaries use sidearms chambered for the 9×19mm Parabellum cartridge, but the effectiveness of the 45 ACP cartridge has ensured its continued popularity with large caliber sport shooters, especially in the United States. In addition, select military and police units around the world still use firearms firing the 45 ACP. In 1985, the 45 ACP M1911A1 pistol was replaced by the Beretta M9 9mm pistol as the main sidearm of the U.S. military, although select Special Operations units continue to use the M1911A1 or other 45 ACP pistols.
Rounds are available from 68 grains to 300 grains (4.4 g to 16.5 g) with a common load being the standard military loading of a 230-grain (15 g) FMJ bullet (for comparison, the most common 9mm load is 115 grains (7.5 g), quite literally half the weight). Specialty rounds are available in weights under 100 grains (6.5 g) and over 260 grains (16.8 g); popular rounds among reloaders and target shooters include 185-grain and 230-grain (12 g and 15 g) bullets. Hollow-point rounds intended for maximum effectiveness against live targets are designed to expand upon impact with soft tissue, increasing the size of the permanent cavity left by the bullet as it passes through the target.
Tracer ammunition for the 45 ACP was manufactured by Frankford Arsenal and by Remington Arms. This ammunition was available to the United States Border Patrol as early as 1940 and was used through World War II for emergency signalling by downed United States Navy and Marine Corps air crew. Tracer ammunition was identified by painting the bullet tip red.
Most ammunition manufacturers also market what are termed "+P" (pronounced "plus P", designating overpressure ammunition) loadings in pistol ammunition, including the 45 ACP. This means the cartridge is loaded to a higher maximum pressure level than the original SAAMI cartridge standard, generating higher velocity and more muzzle energy. In the case of the 45 ACP, the new standard cartridge pressure is 21,000 psi (140 MPa) and the SAAMI 45 ACP +P standard is 23,000 psi (160 MPa). This is a common practice for updating older cartridges to match the better quality of materials and workmanship in modern firearms.
The terminology is generally given as "45 ACP +P" and sometimes, but not always, appears on the headstamp. These cartridges have the same external dimensions as the standard-pressure cartridges and will chamber and fire in all firearms designed for the standard-pressure loadings. The inner dimensions of the +P cartridge are different from the standard-pressure cartridge dimensions and thus allows for higher pressures to be safely achieved in the +P cartridge. If +P loadings are used in firearms not specifically designed for them, they may cause damage to the weapon and injuries to the operator.
Popular derivative versions of the 45 ACP are 45 and .460 Rowland. The Super is dimensionally identical to the 45 ACP; however, the cartridge carries a developer established pressure of 28,500 psi (197 MPa) and requires minor modification of firearms for use. The Rowland operates at a developer established 40,000 psi (280 MPa) SAAMI and may only be used within a select group of firearms significantly modified for this purpose; the Rowland case is 0.057 inches (1.4 mm) longer specifically to prevent it from being chambered in standard 45 ACP firearms. Brass cases for each of these cartridges carry the applicable name within the headstamp. The Super provides approximately 20% greater velocity than the 45 ACP +P; the Rowland approximately 40% greater velocity than the 45 ACP +P.
“There is little to no noticeable difference in the wound tracks between premium line law Auto enforcement projectiles from 9mm Luger through the .45 Auto.”
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