DUH.. They had the pistol they needed in the 45acp round.
Never should have left it.
Wrong! The U.S. military has never chambered any of their weapons with the .45 ACP round. Not the Model 1911 pistols, not the 1917 Revolvers, not the Thompson sub machine guns, not the M3 'Grease Gun' series, not the Liberator Pistol, not the Riesing sub machine guns, etc. The all were chambered for the Cartridge Ball, Caliber 45, Model of 1911.
The .45 ACP, .45 Auto and .45 Model 1911 are distinctly different cartridges, all built to different Technical Data Packages with different acceptable pressure limits, tested by completely different equipment and techniques. The fact that they are dimensioned similarly is irrelevant to safety considerations. Same is true of .223 Rem & 5.56mm, 9mm Luger and 9mm Luger+P, etc.
The American National Standards Institute (ANSI) and the Sporting Arms and Ammunition Manufactures Institute (SAAMI)are the authorities on cartridge names and designations.
SAAMI obsoleted the ACP designations prior to WWII. Where have you been? Granted, most gun writers and off-shore manufactures haven't gotten the word either.
Many ill informed folks want to argue this issue by pointing our how many others are ignorant of history and reality. Matters not. To paraphrase James Carville, "It's the headstamp stupid!" Or the designation on the firearm.
It is no more appropriate to address a married woman by her maiden name than it is to use obsoleted cartridge designations.