calibers?

[Jim Watson]

The problem is not that there is not a consistent system of cartridge and caliber nomenclature, it is that there are several and you have to be sure which (if any) is in use at the time. I know of no way to handle it except by rote memory. A copy of Cartridges of the World and a handloading manual will help a lot.

Just think, .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Remington, .223 Remington, .224 Weatherby, and .225 Winchester ALL shoot .224" diameter bullets.
But .22 Remington Jet, (early) .22 Hornet, and .22 Savage High Power don't. And they don't shoot the same size bullet as each other, either.
Isn't advertising wonderful?

Oh, by the way, in England a .450-400 is NOT a .45 caliber; it is a .40. They refer to a cartridge necked down from a larger one with the old diameter first and the new diameter second. We do it the other way 'round in the colonies, like .22-250.
And you don't even want to think too hard about the EM2 and its final cartridge, the .280/.30 (7mm bullet, .284" rounded down to .280 in a cartridge case with the head diameter of a .30 (US .30-06 or .308) as opposed to earlier versions with smaller head diameter.)

 

[MachIVshooter]

The caliber actually has to do with the diameter of the interior of the barrel. Here in the United States, with the customary system, we measure the distance from one groove to the opposite groove, the largest measure of the barrel interior diameter.

Wrong. If we measured groove to groove, the .50 BMG would be a DD.

Cartridge nomenclature is usually only an approximate representation of bore diameter. You cannot outline a rule of thumb because there are more exceptions than adherences. All American .30 caliber cartridges currently manufactured use .308" bullets, but most of them are labeled as .30 this or .300 that, with the exception of the .307 Win, .308 Win., .308 Marlin express and 7.82 Lazzeroni cartridges.

RLsnow:

Your best bet is going to be to pick up a copy of Cartridges of the World. By the time we all get done hashing out the bullet diameters, case lengths and inch to metric conversions here, this thread will be 37 pages long.