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How to Convert Rifle Calibers From Metric

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
- 4
  
  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.
- 5
  Divide the millimeter by the conversion factor 25.4.
- 6
  
  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.

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