How to Calculate Rifle Calibers

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There are a wide variety of surplus military rifles and older-model rifles available for purchase on the civilian market. In many cases, these rifles are not marked with a caliber at all, are marked with archaic nomenclature or have been refitted to accept a different caliber than originally designed. Determining the caliber of these rifles is important, both for safety reasons and for purposes of determining their value as collector's items. The process for determining the bore size of a rifle is called "slugging" and can be accomplished with a moderate amount of preparation.

Items you will need

  • Calipers

  • Hardwood dowel slightly smaller than bore diameter, 4 inches longer than the barrel of the rifle

  • Lead fishing sinker slightly larger than bore size

  • Gun lubricant

  • Old towel

  • Rubber mallet

  • Clamps

  • "Cartridges of the World" or similar reference manual

Cycle the action of the rifle several times while pointing the gun in a safe direction to verify that the gun is completely unloaded. Open the action and visually inspect it to make sure that it is free from live ammunition.

Clean the gun thoroughly.

Spread the towel on the floor over your work area. This will allow you to catch the lead sinker when it drops, and will also protect the floor from damage by lubricants or solvents in the gun.

Set the rifle in a vertical position with the muzzle facing the ceiling and the butt resting on the towel. Clamp the rifle to a vertical post (such as a table leg or chair) to keep it upright during the slugging process.

Coat the inside of the barrel and the outside of the sinker with a light layer of lube.

Gently tap the sinker into the barrel using the rubber mallet, until the top of the sinker is flush with the end of the barrel. You should see a little bit of lead shaved off of the sinker as it goes in--this ensures that you have a good fit.

Using the dowel rod, tap the sinker the rest of the way through the barrel until it falls out the chamber.

Measure the outside of the sinker with the calipers. This measurement will give you the diameter of the barrel. Repeat the measurement two or three times for consistency.

Consult a reference manual, such as "Cartridges of the World," to determine the caliber of your rifle based on the bore size you determined in Step 8.


  • Like all firearms, rifles are deadly weapons and should be treated with caution. Be sure to consult your owner's manual and follow any safety guidelines it outlines.
  • Attempting to fire an incorrect cartridge in a military surplus rifle can be dangerous and may result in injury or death. Additionally, many military surplus firearms may no longer be safe to fire. Consult a competent gunsmith about your surplus rifle's safety before firing.


  • Always keep the rifle pointed in a safe direction.
  • Throughout the slugging process, keep your finger off the trigger and out of the trigger guard of the rifle.
  • Understand that there is a difference between "caliber" (the diameter of the barrel) and the cartridge your rifle fires. Consult any markings on the rifle, as well as reference manuals such as "Cartridges of the World," before firing any cartridge in your rifle.