| The .45 Automatic Colt Pistol (hence ACP) cartridge (actual diameter .452 inch) was introduced in 1905 but only became widely popular when John Moses Browning and Samuel Colt designed and produced the semiautomatic pistol which was to become the Government Model 1911. The m1911 (so called because it was adopted in the year 1911) was chambered in .45 ACP after the .38 Long Colt which was until then the standard G.I. sidearm proved to be ineffective against an extremely determined attacker. These "extremely determined attackers" were mostly moro insurgents in the Philippines and at times were repeatedly shot with the .38 LC or sometimes even with the .30 Krag (a rifle round) but still were able to continue charging and injure/kill american soldiers. Although these moros would eventually die due to the blood loss brought about by multiple gun shot wounds, the non-instantaneous incapacitation proved to be fatal for american troops.
A quote from www.sightm1911.com
It was the cartridge tests conducted by Thompson and Major Louis Anatole LaGarde of the
Medical Corps in 1904 at the Nelson Morris Company Union Stockyards in Chicago that
resulted in the adoption of the .45 caliber as the official U.S. Army handgun cartridge.
They tested various calibers on live cattle, deer, and human cadavers to determine the
best load. From these tests it was determined that the .45 was the most effective
cartridge for a handgun, but with reservations. In their report, they state:
"the Board was of the opinion that a bullet, which will have the shock effect and
stopping effect at short ranges necessary for a military pistol or revolver, should have a
caliber not less than .45". But they also said, "...soldiers armed with pistols
or revolvers should be drilled unremittingly in the accuracy of fire" because most of
the human body offered "no hope of stopping an adversary by shock or other immediate
results when hit."
In response to the Ordnance Board's specification, Browning designed the .45 ACP for the pistol he was submitting to the board. Browning's first loading was a 200 grain bullet running at 900 feet per second, but the Army wanted a larger bullet. Browning responded with the loading we have today, a 230mgrain bullet with a muzzle velocity of 830 feet per second.
The .45 ACP fought in every major conflict and continues to do so despite of the fact that the standard US Armed Forces sidearm is now the M9 Beretta 92FS chambered in the anemic 9mm NATO.
Should you ever consider to carry a gun for self defense the .45 ACP will never let you down as long as you do your part. Shoot and hit the bad guy. Today, as it was, and ever shall be, the .45 ACP is still the perfect cartridge for shooting damn dirty moro insurgents. | 
