How to Convert Rifle Calibers From Metric

Caliber and millimeter both refer to bullet diameter, but not cartridge length.

Gun collecting is rife with different means of measurement, and two of the most common are caliber and millimeter -- both of which refer to barrel diameter. Understanding bullet diameters between millimeter and caliber requires a simple calculation using the conversion factor, 25.4.

Things You'll Need

• Calculator

Instructions

Calculate Caliber to Milimeter

1. 

   The British Bren LMG fires .303cal ammunition.

   Locate the caliber of the round you want to size on the bottom of the casing or on the side of the firearm. Retain the decimal. For example, 0.303cal.

2. Multiply the caliber by the conversion factor 25.4.

3. 

   This StG 44 rifle fires 8mm ammunition.

   Round the result to the nearest round manufactured in millimeters for comparison or barrel modification purposes. For 0.303cal, the quotient 7.7mm would be closest to the Japanese 7.7x58mm rifle round in width; cartridge length is determined by individual models of ammunition.

Calculate Millimeter to Caliber

1. 

   The men in this image hold Mosin-Nagant rifles, which fire 7.62x54mm ammunition.

   Locate the millimeter of the round you want to size on the bottom of the casing or on the side of the firearm. Retain the decimal. For example, 7.62mm.

2. Divide the millimeter by the conversion factor 25.4.

3. 

   Most American rifles in World War II fired .30cal ammunition, including the famous M-1 Garand.

   Round the result to the nearest round manufactured in millimeters for comparison or barrel modification purposes. For 7.62mm, the quotient is .30cal, which means that rifles firing 7.62mm ammunition will have the same barrel width as .30 caliber rifles.

Tips & Warnings

• For collectors in the United States, the caliber or millimeter of the firearm in question will be stenciled into the side of the barrel by the Bureau of Alcohol, Tobacco and Firearms (BATF).

• The caliber or millimeter of the cartridge does not indicate cartridge length. The nomenclature sometimes states the length in cases where a rifle fires a common width, e.g., 7.62x54, 7.62x39 or 7.62x25. All refer to different kinds of ammunition.

• Determining caliber length is much less refined; many may think that the .06 in 30.06 refers to the length of the cartridge, but it actually refers to the year of design, 1906.

• Some caliber nomenclature uses alphanumeric designations to differentiate bullets, for example, .50BMG versus .50AE and .50 Beowulf each refer to different cartridge lengths.

• Never attempt to use a different model of ammunition than was intended for your firearm, even if the bore is the same width and the cartridge fits into the chamber. Always refer to your user manual before attempting to use an ammunition other than what is stated on the weapon itself. Some weapons are not intended to fire heavier grain loads, and firing a higher grain cartridge can cause gun malfunctions.

References

• ATK: The Difference Between .223 and 5.56 Military Cartridges
• Maryland Metrics: Metric Conversion Factors

Resources

• War Wheels: Weapon Measurement Conversion Chart