Posted by on 4/8/2022 to
Back in the day, like way back in the 1800s when rifling was becoming more common, calibers were starting to emerge. Caliber is not determined from a bullet or cartridge; the caliber is determined from the barrel. Specifically, caliber is the internal diameter or bore of a gun barrel. Bullets and cartridges are then made to fit the bore of the gun barrel, and that is why they are labeled as cartridges of a certain caliber. The measurements of the bullet are very close, but slightly larger, than the measurements of the bore of the gun barrel. This press fit kind of interface is necessary to push the bullet into the rifling to provide spin and maintain adequate pressure behind the round.
There are a lot of calibers out there that range in size and differ in use. The full name of a cartridge can also denote the amount of gunpowder, the weight of the bullet, the shape of the cartridge, or even the name of the person or company that designed it. You can have a cartridge with a larger bullet but a smaller caliber than another cartridge. Caliber does not directly correlate with a particular bullet size, though it may correlate to a range of bullet sizes.
These are very precise, down to thousandths of an inch. Calibers can be used in terms of a cartridge because of how it is measured. However, the cartridge is designed to work in the caliber and not the other way around. The main thing you need to know about calibers is that the cartridge can be safely fired from the matching caliber's chamber and that it will safely make its way down the barrel, traversing the rifling grooves creating spin, which will lead to greater accuracy. A tight yet accurate fit is essential since if it is too loose it will not get the desired spin and may kind of knock around the barrel on its way out and if it’s too tight it could lead to explosions and the bullet not even making it out of the barrel. Accuracy is so important that being off by even just a few thousandths of an inch is too much.
To many people, caliber is synonymous with size, though the bigger number does not always mean the caliber is bigger. A 9 MM is bigger than a .22. There are two different measurement systems at play here, Metric and Standard. A .22 is .22 inches wide, while a 9 MM is 9 millimeters wide.