45ACP and 45 Colt are the same caliber.
Look up the word "caliber" in your Funk and Wagnalls.
"Caliber" is the dimension of the barrel bore diameter.
Indeed, but the bullet diameter specifications are:
45 ACP - .451 inches
45 Colt jacketed - .451 inches
45 Colt lead - .452 inches
45 Schofield - .454 inches
454 Casull (jacketed) - .451 inches !???!
FYI the 45-70 bullet is .458 inches in diameter
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