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# How to Convert Rifle Calibers From Metric

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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.

## Step 1

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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.

## Step 2

Multiply the caliber by the conversion factor 25.4.

## Step 3

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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.

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.

## Step 1

••• NA/Photos.com/Getty Images

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.

## Step 2

Divide the millimeter by the conversion factor 25.4.

## Step 3

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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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