![]() The relationship between the meter and the millimeter is constant however. The definition of the meter has changed over time, the current definition being based on the distance traveled by the speed of light in a given amount of time. It indicates one thousandth of the base unit, in this case the meter. History/origin: The milli- prefix is one of many metric prefixes. It is defined in terms of the meter, as 1/1000 of a meter, or the distance traveled by light in 1/299 792 458 000 of a second. Millimeterĭefinition: A millimeter (symbol: mm) is a unit of length in the International System of Units (SI). It is also sometimes used in Japan (as well as other countries) in relation to electronic parts, like the size of display screens. Another version of the inch is also believed to have been derived from the width of a human thumb, where the length was obtained from averaging the width of three thumbs: a small, a medium, and a large one.Ĭurrent use: The inch is mostly used in the United States, Canada, and the United Kingdom. One of the earliest definitions of the inch was based on barleycorns, where an inch was equal to the length of three grains of dry, round barley placed end-to-end. There have been a number of different standards for the inch in the past, with the current definition being based on the international yard. History/origin: The term "inch" was derived from the Latin unit "uncia" which equated to "one-twelfth" of a Roman foot. ![]() There are 12 inches in a foot and 36 inches in a yard. An inch was defined to be equivalent to exactly 25.4 millimeters in 1959. Definition: An inch (symbol: in) is a unit of length in the imperial and US customary systems of measurement. Common Handgun Calibers Converted to MM and Inches Handgun Cartridgeįortunately, understanding the conversions between metric and imperial for handgun calibers is quite a bit easier because there are far fewer common pistol cartridges than there are rifle cartridges.įor the most part, handgun calibers are either 9, 10, 11, or 12mm. 30 caliber cartridge, which isn’t actually correct at all.Īs you can see, the naming conventions of cartridges is complicated because ammo manufacturers like using clean, even numbers and to ignore the actual measurements.īecause the table above has to make some generalizations about particular cartridges, I’ve included the table below which lists each rifle cartridge separately so you can see the specific bullet diameter in inches and its exact metric conversion. The difference is that one measurement is the distance between the grooves (the cut out portions of the rifling in the barrel, and another measurement uses the measurement to the lands (the raised portion of the rifling). Converted to metric, that’s 7.82mm, but most people would refer to it as a 7.62 because of its military distinction. 308 Winchester is so named because the bullet is. ![]() That’s because the naming conventions for many of the most popular cartridges bend the rules of measuring in order to come up with a nice sounding name.įor example, the good old. There are some things in that table that just don’t seem right. If you’re like me, looking at the table above starts to make your head spin. Rifle Cartridge Caliber to Metric Conversion Table Rifle Caliber Name The manufacturers of ammunition have done us no favors in making this easy to understand, but I’ve put together this handy resource so you can quickly see the equivalent naming conventions between mm and inches (caliber). This page will explain the problem as clearly as possible. Many shooters are confused because there are two different systems for measuring the bore diameter for a cartridge: (1) the caliber system measuring the diameter of the bore of the barrel in inches, and (2) the same measurement, but in millimeters. ![]()
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