The caliber is the same as the bore which is the diameter across the lands or the diameter before cutting the rifling. The diameter across the groove bottoms is the groove diameter. Neither of which have anything to do with the cartridge's designation.
Any attempts to link actual measurements to cartridge names will lead only to madness. Consider the 44 Russian/Special/Magnum and the 445 SuperMag as an illustration of the lack of any rhyme or reason. All but the first have a 0.429 groove diameter, and are around 42 caliber or bore. Now consider the 44 S&W which has a larger groove diameter. Then there is the 44 Colt which has still another diameter.
As for the 45 Colt question. First came the 45 Colt.
Next came the 45 S&W, nicknamed the 45 Schofield. This created a problem for the army in that the 45 S&W had a wider rim which allowed only three to be loaded in the Colt SAA.
To get around this the government developed the 45 Government, nicknamed the 45 Short Colt wich had the 45 S&W case length and the 45 Colt rim. This cartridge was sold on the civilian market labeled as the 45 Short Colt.
Next came the 1909 Colt 45 chambered in the Colt New Service Model 1909. This cartridge had the 45 S&W rim width and a case about 0.010 inch longer than the 45 Colt.
Finally came the 45 ACP which was originally intended to duplicate the ballistics of the 45 Government or 45 Short Colt. The 45 ACP case is shorter than the 45 Short Colt case.
There was never a cartridge properly called the 45 Long Colt although during the time when the 45 Short Colt was on the dealer's shelves I am sure people called the 45 Colt the 45 Long Colt to distinguish it from the 45 Short Colt.
Now days we encounter store help who wouldn't know whozzit from whazzit no matter what the history was.
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