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Caliber, as measured in inches, means the 9mm is a .38 caliber bullet and this is a .38 of an inch in diameter. Various manufacturers have different tolerances in the thousandths of an inch. Some of the generic equivalents of the more commonly used 9mm rounds are: 9mm = .38, .380, or even .357 (as in the magnum), however, the caliber of a 9mm round is 9mm. Caliber is the diameter of the projectile. Within every caliber, there are other designations and therefore caliber is based on the internal diameter of the barrel before the rifling is machined or cut into the barrel. The result is that the diameter of the barrel is increased.
9mm also translates to .354-inches which is remarkably accurate for a caliber designation. A 9mm Parabellum-(9mmP) uses a .355-inch bullet or 9.017mm. Compared to the Russian 9mm Makarov which is (9x18mm) which uses bullets of .364-inch. However, the significant difference of the Russian 9mm Makarov is 9.2456mm but it is still classified as a 9mm caliber weapon.
There are several different 9mm rounds in production: 9×18, 9×21, 9mm Largo, 9mm short (Kurtz, Corto, .380), 9mm Luger, or parabellum, etc. The difference is geographic and in some cases, countries deny civilians the use calibers that their armed forces use. For example, in Italy, the 9mm Luger is a military caliber and civilians have to purchase other calibers like the 7.65 Luger.
In the world, the 9mm Luger is the most popular caliber followed by the 9mm Short (Kurtz, Corto, .380).
Understanding What Caliber Means
Caliber designation is nominal and should not be used as a true identifier of actual bullet diameter. I know that is confusing but it is the way it works.
Caliber can be expressed in either inches or millimeters. When talking about small arms, such as rifles and handguns, we say caliber to mean the diameter of the bullet. When it is about artillery pieces, caliber refers to barrel length.
Inches vs Cal
Consider the example of the 16 inch 50cal. Here caliber refers to the length of the barrel. It means the length is 50 times the diameter. To find the barrel length you multiply 16-inches x 50 = 800. Therefore the barrel is 800 inches long.
We also list calibers by the effective inner diameter of the rifling or the diameter of the lands. European gun makers refer to this as the diameter of the grooves or the outer diameter of the rifling. In America, the.30 caliber usually comes in .308 inches bullets. In Europe, the.30 caliber comes in .312-inches. The four-thousandths of an inch makes a difference in function.
A .308 bullet will not seal well in a .312-inch barrel, and this may result in the loss of both energy and accuracy. On the other hand, a .312-inch bullet in a .308-inch barrel will run you into an overpressure condition. The sequence of events should you try this is determined by which parts will fail first.
The shots you get away with before your barrel defects depend upon how your firearm is well-built. Although there is a possibility that the bullet may not even exit the barrel. In the second case, the barrel may or may not succumb to the excess pressure. What this means is that you may get away with a jugged barrel and not ultimately a rupture. However, it will likely inflate like a metal balloon leaving your firearm with a swollen section of the barrel.
Effects of Measurements Rounded to Nearest Thousandths.
Here, you must note that there is a different form of measuring calibers for people in Europe and the USA. Therefore the importance of understanding rifling nomenclature will make you adjust to the differences in measurement. A clear distinction is that the American standard bullet groove diameter uses inch convention. The European standard bullet land diameter uses metric convention.
Variations in Equivalent Calibers
The difference explains the variations in equivalent calibers of the bullet should you use calipers to determine the size. The caliber may still function in the same weapon. An example of interchangeable rounds is the 5.56mm NATO which is a .22 caliber bullet but measures .223. Therefore, you can fire a .223 Remington cartridge from a weapon designed to fire the 5.56 NATO round. The reverse is not true and is not safe. Other 9mm pistol cartridges that are not quite as common include, 9×17, 9×18, 9×21, 9×23, and 9x25mm.
The various cartridges also usually have a name associated with them. The 9x19mm is also often known as the 9mm Luger, 9mm Parabellum, and 9mm NATO. The 9×17 is the European equivalent of the .380 automatic.
The 9×18 designation is applied to two different cartridges, and the most common one is the Soviet 9x18mm Makarov which is not considered a true 9mm as it has a larger bullet diameter and bore. The other one is the 9×18 Ultra, also referred to as the 9×18 Ultra Police.
The 9x21mm is known as the 9×21 IMI (Israeli Military Industries) but it is also called the 9x21mm Winchester. The 9x21mm is intended for use where it is illegal for civilians to own firearms chambered for military cartridges like the 9x19mm.
The 9x23mm is known as the 9mm Largo and is an old military pistol cartridge from the early 20th century. The 9x25mm also goes by 9x25mm Dillon and was developed for competition use so that IPSC competitors could make what is known as a ‘major power factor’ with a 9mm pistol and gain a competitive advantage over other 9mm cartridges.
There are other 9mm cartridges out there, but they are fairly large for a rifle as most are suitable for a smaller caliber.
Different 9mm Calibers of Pistols that are Similar
The 9mm Kurtz or 9mm short is a .380 or 9 X 17mm. It is the smallest of the 9mm calibers available in the market. Then the 9 X 18 or the 9mm Makarov. The