That designation was obsoleted by SAAMI in 1940 and replaced with ".45 Auto". SAAMI and the American National Standards Institute are the authorities, not a bunch of wannabe gun nuts.
Where have they been? Reading too much from gun writers, the majority of whom gained their entire education on the subject by reading other gun writer's drivel"
The U.S. military has never chambered any of their type classed weapons for the .45 ACP or .45 Auto cartridges. Not the 1911 serious of pistols, not the 1917 Revolvers, not the Thompson sub machine gun series, not the M3 Grease Gun series, not the Liberator Pistol, not the Reising submachine gun, etc. They were all chambered for the Cartridge, Ball, Caliber 45, Model 1911.
And for the neophytes, the .45 ACP, .45 Auto, .45 Auto+P and Caliber 45 Model 1911 are different cartridges even though most dimensions are similar. Each is manufactured and acceptance tested with completely different Technical Data Packages. The pressure specifications are different and not even tested with the same type equipment and procedures. The numbers are not interchangeable. Every box of ammunition made by SAAMI members and other responsible manufactures carries some form of the warning, "Only use ammunition that exactly matches the markings on your gun" Read the headstamp stupid! And don't waste your and other's time with silly lists of the exceptions you can find that document ignorant or irresponsible manufacturers. Some folks are just real slow in getting the word. Lift yourself above that level.
Good day.
There now!
Don’t you feel so much better?