What Exactly Is It?
Specifically, caliber is a diameter measurement. It could be the measurement of a bullet’s greatest diameter. It could be the measurement of the distance between a barrel land to land, or groove to groove. More generally, it is the designation given to different cartridges. I have put it this way because, as we shall see, the figure for a particular cartridge may not directly correspond to any particular measurement. It is quite difficult to truly separate a look at caliber measurement from cartridge nomenclature. I hope that by taking the issue in two parts, the confusing world of cartidge nomenclature will be a little more navigatable.

European vs. American
The European caliber designation uses the metric system. The metric system is a measurement system based on the number 10, unlike the English system which uses all sorts of handy, but arithmetically challenging units. For instance, in the English system, a foot is composed of twelve inches, a yard of three feet, and a mile of 5280 feet. In the metric system, a meter is composed of one hundred centimeters, and a kilometer is a thousand meters. Moving between large measurements and small measurements in the metric system is done by dividing or multiplying by some number of tens.

In the metric system, a handy small unit of measurement is the millimeter, which is a tenth of a centimeter, and hence a thousandth of a meter. Readily, one can deduce that a nine millimeter object is nine tenths of a centimeter. The European method for determining caliber is to measure in the metric system.

The American (and to a great extent the British) method of measuring caliber relies on the English measurement system, with a twist. Anyone familiar with machines knows that measuring small items as fractions of an inch can be less than convenient. Machinist often use hundredths or thousandths of an inch, instead of fractions of an inch. This means that the measurement of a small item is expressed in a decimal format, and thus is easy to add, multiply, and subject to a calculator or computer. Americans tend to measure their bullets in thousandths of an inch. This confuses many newcomers since most people are not used to thinking of inches in this way. Rulers displaying less than an inch measurements rely on dividing an inch into eight, sixteen, thirty-two,or sixty-four pieces instead of a hundred or a thousand pieces.

Nominal vs. Actual
Nominal means in name only. Someone with nominal power may hold the title of president, but have no governing power. With caliber determination, we find that there is a nominal caliber for a particular bullet, and then the actual caliber.

An example of a caliber designation that is accurate is the .357 Magnum cartridge. The bullet for the .357 Magnum cartridge is actually 357 thousandths of an inch in diameter. Compare this is a .44 Magnum shell, which has a bullet which is 429 thousandths of an inch.

Sometimes, the minute changes in tolerances that are allowed in bullet manufacturing can account for nominal versus actual caliber designations. The 7.92MM Mauser is often referred to as the 8MM Mauser because 7.92 millimeters is just about 8 millimeters, with .08 millimeters of difference. Bullet manufacturing tolerances will often account for a large portion of the .08 millimeter difference.

The Magnum Confusion
Without getting into cartridge designation, let’s say a few words about the term magnum.

Magnum means large. More appropriately, something can be termed magnum when there is a smaller version of that something. One can have a magnum version of a regular champagne bottle. There is a regular sized champagne bottle, holding a fifth of champagne, and then there is a larger bottle holding still more champagne, a magnum sized bottle. Similarly, in the cartridge world, magnum refers a shell that is more powerful than a comparable shell. The .44 Magnum is more powerful than the .44 Special. The two shells are completely comparable except that the .44 Magnum cartridge is more powerful. It also happens to be longer. It doesn’t have to be longer, but it is in order to prevent a user from inserting a .44 Magnum shell into a gun built to fire .44 Special rounds. The change of length is done solely for safety reasons.

I have brought up the concept of magnum here in order to show that it has nothing to do with the size of a bullet: it is only a designation given to a complete cartridge. More specifically, it really relates to power and not to the size of the shell, though one will most always find that magnum versions of shells are longer than the non-magnum shells.

More about the use of the word magnum when we finally look at cartridges.