You stopped reading the Wiki article too soon.

Scroll down to the section 1.2, "Caliber as Measurement of Length".

Actually, most people when asking what caliber, are asking what is that firearm chambered for.

Most non gunnuts have non clue that many of our cartridges are based on the same hole drilled in the barrel. Likewise, many just assume a bigger number means a more powerful gun. I had someone insisting to me that I was carrying a howitzer when I said I carried a .45, because Dirty Harry carried a .44, there fore, my .45 had to be much more deadly.

That's like saying a 9mm makes a bigger hole than a .380.

Then you have to explain that a .357 hole isn't any smaller.

Actually, you end up explaining that a .357 hole is LARGER!! (Ties knots in yer brain, don't it?)

Please... please... don't tell me you think 0.002" matters

Sounds like a kink is starting in Charlie's brain!

No, I don't think .002" matters....but since we are discussing a technical issue, it is the "technical" truth. And in some ways, it does matter. For instance, those revos chambered for .357 diameter bullets that come with a spare cylinder chambered for 9mm; the 9s usually don't group as well because of that .002" difference. (Not that such is a real world consideration in a snubby meant for use at close ranges.)

My father left me some old military manuals, one of which was from the Coast Artillery. In pendantic pre-WWII army-speak: calibre is bore diameter in whatever measurement system you're using. The standard usage was to put the term before the numerical designation, as in a calibre .50 machine gun. I expect this usage was limited to what were considered small arms. Caliber is the length of the gun tube expressed in bore diameters, as in a 5 inch, 50 caliber gun.

Most people-especially Americans-don't bother with esoteric spelling differences. You generally figure out what they're talking about from context.

Actually Ed, it does sometimes. Depends on whether you're talking aboout a 9mmParabellum(.355) or a 9mmMak(.360). Just for clarity, I usually try to use the decimal points where needed as that tells one automatically that it's being referred to as Caliber expressed in the decimal system. For example, .357Magnum(True .357caliber or, diameter). What really confuses the uninitiated is the fractal Euro-designators such as 7.62x39mm or 7.62x54Rmm or 7.62x51mm(.308) or 7.62x63mm(30-06). the last two being Euro-designators for American Cartridges.

Oh, what a tangled web we can weave!

Hmmm...
I remember this too.
It'd be a good idea to go back to this system to eliminate the confusion which began this thread.
One could say "calibre 9mm" without feeling as if it were an oxymoron.

There's still that spelling thing, though...

Or should it be pronounced "cali-BREE" (whatever) for small-arms and cali-BER for naval artillery?

"Calibre" is from the English spelling style, whilst (also English usage) "caliber" is from US traditions.
The pronunciation remains the same, which does lead to confusion.
Perhaps the spelling need not change. The mere position of the word in the term should be enough.
To wit: For small arms it'd be "caliber 9mm," or "caliber .30-'06," while for artillery it would read "caliber 75mm, 20 calibers."