So, what do you have against the term ACP? It stands for Automatic Colt Pistol. The .45 ACP is one of a series of ACP cartridges the John Moses Browning developed for his pistol designs in the early 20th century. These include the .25 ACP, the .32 ACP, the .380 ACP, and the .38 ACP.
“the John”should be “that John”
He did not help his tirade much with that.....
It was changed on referenced cartridges by SAAMI over 70 years ago! SAAMI cartridge designations have been adapted by the American National Standards Institute. Where have you (and so many ill informed gun writers) been? It is no more correct to use obsolete cartridge designations than it is to address a married woman by her maiden name.
In addition, 45 ACP, Caliber 45 Model of 1911, 45 Auto and 45 Auto+P and are not the same cartridge. Period! Each was or is manufactured to a different Technical Data Package. Pressure testing on the first three are not even with the same equipment or pressure testing protocol.
That the subject cartridges may be interchanged in some pistols without catastrophic damage is irrelevant. Every SAAMI member (and most responsible manufacturers world wide) have some variation of the following warning on every box of ammunition, "Use only in firearms originally chambered for this cartridge and so inscribed on the barrel".
To paraphrase a renowned political prudent, "Its the Headstamp, Stupid"!