The reason for the mistake made in JJ was as noted above - due to an incorrect understanding of the words and how they were applied in period:
The term caliber or calibre (French/Spanish spelling)was originally used as a synonym for gauge or bore size - all three originally were based on balls per pound for that diameter bore. One researcher (Diez) suggests the term caliber, whihc is of uncertain origin, may be derived from the Latin quá librá = of what weight. Applied to the bore of a gun it would be be determined by the weight (and consequent size) of the bullet i.e. gauges, bore, balls per pound.
Bore size by balls per pound or gauge was the more commonly used term by the Brits (and later Americans) but even they used the three terms interchangeably up until about the mid-1800's or so. But you have to read original pre-1850 or so sources to find the terms used thusly.
The use of the word caliber for the decimal equivalent of the bore size in inches/fractions of an inch i.e. .54 caliber instead of 32 gauge/bore/balls per pound began circa the 1850's (at least that's as early as I have found the term being used to describe the bore diameter in inches). One example is an advertisement for Colt Navy pistols from 1859 - it describes the bore in three different ways:
Balls per pound/gauge of round ball
Balls per pound/gauge of conicals (they are heavier than the balls)
The diameter of the bore in inches: in this case .375"
Based on period research, IMO it was the wider spread use of conicals that initiated the change in common meaning of caliber/calibre from the same as gauge/balls per pound to that of the diamter in inches.
So IMO what the author and writers did wrong was to make an assumption based on incorrect data. Somewhere in the period research done by the author he came across the 30 caliber designation and ASSUMED it meant the bore size in inches (as was common in the mid-20th Century when the book was written) rather than the correct designation of balls per pound.
Thus in both the author's fertile imagination (Crow Killer is decent historical fiction/folklore at best - it is a poorly researched biography) the term caliber meant diameter in inches and not gauge. Then later the script writer's just followed suit and made the same error - plus the whole "he wanted a 50 caliber instead of a 30 caliber" is a good sub-plot line that adds interest to the story no matter how grossly wrong it is.
Here's a pretty good site for those interested in Johnson and the real story as well as the books and the movie:
http://johnlivereatingjohnston.com/[/quote]
What he said...
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Eterry