The.45 ACP (Automatic Colt Pistol), also known as.45 Auto,.45 Automatic, or 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 experienced in the Moro Rebellion in places like Sulu. The issued ammunition,.38 Long Colt, had proved inadequate, motivating the search for a better cartridge. This experience and the Thompson–LaGarde Tests of 1904 led the Army and the Cavalry to decide that a minimum of.45 caliber was required in a new handgun cartridge.

The standard issue military.45 ACP round uses a 230-grain (14.9 g) round nose projectile that travels at approximately 830 feet per second (250 m/s) when fired from a government-issue M1911A1 pistol. It operates at a relatively low maximum chamber pressure rating of, compared to  for both 9mm Parabellum and.40 S&W, which due to a low bolt thrust helps extend the service lives of weapons. Since standard-pressure.45 ACP rounds are subsonic when fired from handguns and submachine guns, it is a useful caliber for suppressed weapons to eliminate the sonic boom.

Design and history

During the late 19th century and early 20th centuries, 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 standard-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 animals in the Chicago stockyards, resulting in their declaring that the.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  bullet at, but after a number of rounds of revisions between Winchester Repeating Arms, Frankford Arsenal, and Union Metallic Cartridge, it ended up using a  bullet fired at a nominal velocity of. 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 Luger pistols chambered in.45 ACP, withdrew from testing after the first round of tests, for unspecified reasons.

During the second round of evaluations in 1910, the Colt design passed 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  round with a bullet weight of. The first production, at Frankford Arsenal, was marked "F A 8 11", for the August 1911 date.

The cartridge was designed by John Browning for Colt, but the most influential person in selecting the cartridge was Army ordnance member General 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.

Military cartridges

U.S. military

The "T" (trials) designation was used for the experimental version of a cartridge and "M" (model) is used for the accepted and standardized version.
It came in either 24-round revolver ammo cartons, containing eight 3-round "half-moon" clips (1917-1945), pre-war 20-round cartons (1911-1942), or wartime 50-round cartons (1942–present). The M12 and M15 shotshell rounds were packed in 20-round cartons sealed in heat-sealed foil-lined Kraft paper.     
Caliber.45 ball M1911 was the standard full-metal jacketed lead-core bullet.     
Caliber.45 dummy M1921 has a hole drilled through the case and does not have a primer.  
Caliber.45 blank M9 has a tapered case and does not have a bullet.
Caliber.45 shot M12(T23) (1943-1944 ) was a survival round with a round-nosed red wax paper projectile containing 118 pieces of No.7  birdshot. It was issued in USAAF survival kits to allow pilots and aircrew armed with the Colt M1911A1 to use it for hunting small game. The shotshell was a little longer than a standard round, so the operator had to load it in the action individually. It was extracted after firing by removing the magazine, pulling back the slide, and pushing down on the case until it fell down the magazine well. Reports showed that the paper projectile was affected by humidity and would swell or break apart. It was made limited standard until replaced by the.45 M15 shot cartridge.   
Caliber.45 shot M15 was an improved survival round loaded with 108 pieces of No. 7 birdshot, with wadding and a vermilion cardboard disc sealing the casemouth. It was loaded and extracted exactly like the M12 shot cartridge. 
Caliber.45 tracer M26 (T30) has a red tip. The round was designed as a short-ranged red flare for use in emergency signalling.

Commonwealth military
"S.A." stands for small arms. The "z" in the designation stands for cartridges loaded with nitrocellulose rather than cordite.
Cartridge, S.A., pistol,.45-inch Colt Automatic, ball (1917) was the British designation used for American-manufactured ammunition. The Royal Navy had purchased a shipment of M1911 pistols in 1917 along with enough ammunition for evaluation, training and service purposes. It was never standardized by the Lists of Changes, but was mentioned in the Vocabulary of Priced Stores. It came in seven-round packets and was manufactured by Winchester. 
Cartridge, S.A.,.45-inch, ball Mk Iz (1940–1945) was the designation used for American-manufactured ammunition and proposed British manufacture of.45 M1911 ball. Lend-lease ammunition came in commercial 42-round yellow Winchester or 50-round white Western Cartridge Company cartons. U.S. military-issue ammunition came in 20-round cartons, shifting to larger 50-round cartons in early 1942. It was never manufactured in Britain because it was readily available from American forces.
Cartridge, S.A.,.45-inch, ball Mk IIz (1943) was a variant proposed for the Royal Navy, but never put into production. 
Cartridge, S.A.,.45-inch A. C., ball (1942–1946) was the Canadian designation for their domestically manufactured ammunition for use in the European theater. It came in a plain 42-round carton that mimicked the capacity and dimensions of the yellow commercial Winchester ammunition cartons sold to Britain through Lend-lease.
Cartridge, S.A.,.450-inch, ball Mk IIz (1943–1956) was the Australian designation used for their domestically-manufactured ammunition for use in the Pacific theater. It came in 24-round cartons.

French Union military
Cartouche de 11,4 3mm, Pour Pistolets ('11.43 mm cartridge for pistols'): Balle ordinaire ('ordinary ball') pistol ammunition. Post-war production for use in the pistols and submachineguns given out by the US as military aid. It came in 25-round cartons.

Cartridge dimensions
The.45 ACP has 1.62 mL (25 g H