What are the relationship of gauge, millimeter and caliber in
determining the size of firearms?

Hello

Thanks for the question. In one of life's little coincidences I was
just researching the same thing for use in a story I'm writing.

CALIBER (or calibre) is "usually" a numerical term, without the
decimal point, in a cartridge's name to indicate the bullet diameter.

Alternatively it can be a measurement of the diameter of the gun’s
barrel.
It is described in most glossaries as:  "The internal diameter of a
gun's barrel. This can be measured either in English units or in
metric. The measurement can be taken in a rifled arm either land to
land or groove to groove. E.g., in most US .30-caliber rifles, the
diameter of the bore land to land is .300 inches, while groove to
groove it is .308 inches ( Land is the raised portion of rifling in
the barrel, groove is the recessed portion of rifling)"
Taken from "A Glossary of Firearms Technology"
(http://www.building-tux.com/dsmjd/tech/glossary.htm )

In the US the measurement is usually expressed in hundreds of an inch;
in Great Britain in thousandths; in Europe and elsewhere in
millimetres. That's why you see the calibre of handguns often
expressed in different units.

Just to confuse matters, even in the same country the quoted calibre
does not always indicate the true bore diameter but measure the
diameter of the "outer" barrel.

To quote from "A journalists Firearms Glossary" 
( http://www.freep.com/jobspage/academy/guns.htm )
"Not all calibers indicate bore diameter. The .44 Magnum is not .44
inches in diameter. It is .429 inches in diameter. A .38 Special is
not .38 inches. It is .357 inches. The .357 Magnum, though, is truly
.357 inches in diameter. A 9mm is .355 inches. Some calibers are also
confusing. A .45 Colt is a revolver cartridge. A .45 ACP (Automatic
Colt Pistol) is an automatic cartridge. They are not interchangeable."

Caliber also has an alternative meaning when it relates to the length
of the barrel in larger, artillery, pieces eg an artillery piece may
be described as  5 inch/50 caliber, which would mean it has a barrel
50 times the bore diameter, or 250 inches long.
 
                                         ..o0o..

GAUGE refers to shotguns and is the bore size of a shotgun determined
by the number of round lead balls of bore diameter that equals a pound

For example, twelve lead balls that fit the diameter of a 12 gauge
shotgun equal one pound. The most common sizes of shotgun gauge are 10
gauge, 12 gauge, 16 gauge, 20 gauge, and .410. ( Again, just to
confuse matters, the .410 is the exception in shotguns in that it is
measured as a caliber not a gauge, so the .410 has a barrel .410
inches in width.)

So, using this system, as the gauge goes up, the bore diameter goes
down.  This is the opposite of calibre.

Hope that clears it all up. Anything else, just ask for clarification.

Willie-ga

Google search used
firearms glossary

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Request for Answer Clarification by baron5-ga on 02 Nov 2002 08:27 PST

Then the .410 gauge shotgun is more accurately called a .410 caliber
shotgun, right--or wrong?

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Clarification of Answer by willie-ga on 02 Nov 2002 09:32 PST

Q: Then the .410 gauge shotgun is more accurately called a .410
caliber
shotgun, right--or wrong?

A: Yes, that's right. To be accurate you should say .410 calibre. 
But most people who talk about shotguns are so used to talking in
"gauges" that they refer to it as a .410 gauge.