What is the difference between inches, millimeters and caliber when talking about guns?
In: Engineering
Oh boy, this one is a rabbit hole because there’s no real standards and *a lot* of marketing nonsense.
First, the basics: When a firearm is described as “thirty caliber” (.30) that means that the internal diameter of the barrel is .3 inches. Since the late 1800s most of the world outside of the UK and USA has used metric measurements (millimeters) for the same thing. So a gun that is “nine millimeter” (9mm) has a barrel with an internal diameter of 9mm.
Now comes the confusing stuff, because there is another set of important measurements, cartridge length. For example, the most common 9mm cartridge is 9x19mm, and it is 9mm wide and 19mm long. It is commonly known as 9mm Parabellum or 9mm Luger. It is not interchangeable with 9x18mm (9mm Makarov), 9x20mm (9mm Browning Long), 9x17mm (9mm Kurz), etc.
Imperial measurements play an even dumber game, because they are often just lies. .38 Special is actually .357 inches in diameter, the same width as .357 Magnum. .45 ACP is actually .451″ and .45 Colt is .452″. It gets real silly.
Then you have OLD stuff, which was often measured in caliber and charge. So you get .45-70 Government (.45 caliber, 70 grains of powder) and 30-30 (.30 caliber, 30 grains).
Don’t even get me started on naming conventions for artillery because they are twice as insane.
Caliber is a hybrid measurement based on the inch, used only for guns. It is based on a decimal division of the inch into hundredths. This is different to engineering, which breaks the inch into fractions such as 9/16. Both methods are inferior to the metric system of millimetres.
It’s a measurement of the internal diameter of the barrel of the gun. Caliber uses imperial units as a fraction of an inch. For example, a .40 caliber pistol has a barrel diameter of 0.40 inches. Millimeters is metric, so a 9mm pistol has a barrel diameter of 9mm.
Caliber is a measure of the internal diameter of a gun barrel. Inches are an SAE unit, and millimeters are a metric unit. Either can be used to describe the caliber, based on the manufacturer’s preference.
Caliber = the inside diameter of the gun barrel.
This can be stated in millimeters or inches. The millimeter calibers are easy to understand; 9mm means the inside barrel is 9mm in diameter and so on. If you see something like “45 caliber” this means a 0.45 inch inside barrel diameter.
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