Why does caliber refer to a dimension?
I often get confused when referring to gun sizes. I often say "caliper" when the term used is "caliber". Looking at the defintions, it seems to make no sense as "caliber" has two very different meanings; "caliper" also has two very distinct meanings.
caliber
the quality of someone's character or the level of someone's ability.
a) the internal diameter or bore of a gun barrel. b) the diameter of a bullet, shell, or rocket. c) the diameter of a circular body, such as a tube, blood vessel, or fiber.
caliper
an instrument for measuring external or internal dimensions
having two hinged legs resembling a pair of compasses and in-turned or out-turned points.
I think the confusion comes in the because when talking about "the internal diameter or bore of a gun barrel" one would think a term referring to a "instrument for measuring internal dimensions" would be more closely related than a word whose main meaning is "quality of character".
Is there a reason why "caliber" was chosen over "caliper"?
They are only one letter apart, so did they have a similar origin that eventually became more divided?
The sense of "inside diameter" is probably from the the Arabic meaning of "mold for casting bullets":
Caliber:
- 1560s, "degree of merit or importance," a figurative use from Middle French calibre (late 15c.), apparently ultimately from Arabic qalib "a mold for casting." Arabic also used the word in the sense "mold for casting bullets," which is the oldest literal meaning in English. Meaning "inside diameter of a gun barrel" is attested from 1580s. Barnhart remarks that Spanish calibre, Italian calibro "appear too late to act as intermediate forms" between the Arabic word and the French.
(Etymonline)
Caliber was originally used to mean a unit of weight for cannon shot. So, for example, you might have "12 pounds caliber". Much later it came to mean a bore size, but until about 1800 guns were always sized by the weight of the shot.
The analogy to human characteristics refers to weight, not size. So, someone or something of "high caliber" would be someone weighty or serious.