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.
Latest Answers