How to Measure a Rifle Bore's Diameter

Military surplus rifles like this Mauser can have wide bore diameter variations.

Military surplus rifles do not necessarily adhere to modern standard bore diameters. For example, rifle owners take for granted that ".30 caliber" means .308-inch groove diameter, which is the measurement of the bore including the rifling grooves. But a military surplus .30-caliber rifle can measure anywhere from .307-inch to .317-inch. Any rifle gives best accuracy if the bullet is the same size as the groove diameter, so the owner of a military surplus rifle should measure this dimension and choose the bullet diameter accordingly. This mostly applies to those who hand load their own ammo, since ammunition factories tend to default to the standard sizes.

Things You'll Need

• Rifle cleaning kit
• Gun oil
• Wood dowel, 1/4-inch diameter by 36 inches long
• Wood saw
• Lead fishing weights, egg shaped, assorted sizes
• Grease (automotive bearing grease is fine)
• Mallet
• Shop towels
• Micrometer

Instructions

1. Check that the rifle is unloaded. Clean the bore of the rifle. Once the cleaning patches come out clean, wet a patch with oil and run it through the bore.

2. Cut the dowel into 6-inch sections. Make the cuts as straight as possible.

3. Check that the rifle is still unloaded. Stand the rifle on the floor with the muzzle pointing up. Choose a fishing weight of slightly larger diameter than the rifle bore. The weight should not be able to enter the rifle's muzzle. If the nearest size weight is too small, stand the weight upright on the floor and tap it with the mallet until it expands to a usable diameter.

4. Apply grease to the fishing weight. Set the weight on the muzzle of the rifle. Use the mallet and one of the dowel sections to begin tapping the weight into the bore of the rifle.

5. Continue tapping the dowel down until its end protrusion is within approximately 1/2-inch of the rifle's muzzle. Use a second dowel section and the mallet to continue tapping the first dowel section into the bore. Keep adding dowel sections and tapping them down until the weight falls into the cartridge chamber. Depending on the length of the rifle's barrel, this could take anywhere from three to six sections.

6. Hold the rifle at an upward angle and open the bolt. The weight should follow the bolt out. Retrieve the weight and wipe it clean with a shop towel.

7. Use the micrometer to measure the diameter of the weight. There will be rifling grooves impressed in the weight; measure across the high points of the grooves to determine the proper bullet diameter for that rifle.

Tips & Warnings

• Always double check that the rifle is unloaded every time you pick it up to work on it.

References

• Surplusrifle.com: Slug, Measure and Match