RE: kid question
"Caliber is simply the bullet diameter in inches."
"By the way,bullet diameter is the groove diameter of rifling."
I wish it were that simple! But it ain't.
"Caliber" designation can come fromthe bullet diameter, orfrom measurement of the bore diameter before the barrel is rifled.
For example, a .308 Winchester" fires .308" bullets, but so does a .30/30, a .30/'06, a .300 Savage, a .300 H&H Magnum, a .308 Norma Magnum, a .300 Weatherby, etc., etc. You get the idea - all guns that fire .308" bullets are ".30 caliber" arms!!
As pointed out above, many rounds have been given names that are NOT accurate descriptions of their actual bore or bullet diameter. For example, a .38 Special, which actually fires bullets ranging from .356" to .358" in diameter,is actually a .35 caliber because that was the diameter of the gun's bore before it had rifling grooves cut in it.(Just like the ".357 Magnum"). There are a number of similar mis-named cartridges out there. For example, the ".44 Magnum" actually fires bullets of .429" diameter. The so-called ".38-40 Winchester" is actually a .40" caliber - there are just scads of such mis-named cartridges.I believe we are about the worst (U.S.) at misnaming cartridges, and have been working diligently to create confusion since about 1866. (Well, maybe the Brits are as bad!)
Much of this caliberconfusion is a result of carryover from the days of muzzle-loaders. For example, these so-called ".38's". In the 1800's, the Colt cap & ball NAVY Model revolver was among the most popular. It was a ".36" caliber, which meant that it was bored and rifled so that when the ball emerged from the muzzle, it was around .36" in diameter. But in order to make the gun safe from chain firing (more than one chamer went off at one trigger pull), an oversized ball had to be rammed into the chambers to seal off the powder chasrge under tha ball. These balls were about .375" in diameter. So when the frontstufflers were converted to use metallic cartridges, the first ammo made for the converted guns was loaded with "heel bullets" (like a .22 rimfire is to this day!!) which had a section about .36 caliber loaded into the case, andthe part of the bullet outside of the case, in front of the case mouth, was around .38". Hence, the ".38" designation we still use, despite the fact that when new cartridges were designed for new gunswith bored-thru cylinders after about 1873, the bullets were re-designed and all became .356"-.358" in dameter to fit the new, smaller bores. But we still called them ".38's, and still do! Same thing, more or less happened to the ".44's".
The Europeans are a little better. They have a "system". They name a cartridge by (approximately!!) the bore size of the barrel before rifling grooves are cut, AND the LENGTH of the cartridge case. For example, the "7X57mm" cartridge has a 7mm bore diameter, and shoots bullets around 7.2mm in diameter (.284"-.286") from a cartridge case that is 57mm long. If they added a rim to the cartridge for use in single-shot and combination guns, it would be a 7X57R.