How to Measure Bore Size

Knowing the exact measurement of your gun bore is of crucial importance. Whether you are checking for erosion or pitting in the barrel, gauging the buildup of residue, or checking a muzzleloader or musket for exact dimensions as you prepare your ammunition and loading patches, an accurate measurement is crucial. Ignorance of your exact bore size can not only lead to inconsistent performance and inaccurate shooting, but it can also be dangerous. Fortunately, with a good bore gauge, measuring the bore size takes mere minutes.

Things You'll Need

  • Bore gauge
  • Cleaning rod
  • Cotton patches
  • Powder solvent
  • Gun oil
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Instructions

    • 1

      Unload the gun completely. Remove the magazine, if the gun has one. Work the gun's action, pulling back the bolt to eject any round that might remain in the chamber. Visually check to be sure the chamber and magazine are empty.

    • 2

      Clean the gun completely using the cleaning rod, cotton patches and powder solvent. Douse a patch with solvent and use the rod to slide it up and down the length of the barrel. Grime in the barrel can lead to inaccurate bore measurements, so clean it carefully, running cotton patches down the barrel until they come out clean.

    • 3

      Insert the bore gauge into the muzzle end of the barrel a couple of inches and observe the reading on the dial. Keep the gauge as straight as possible for the entire measuring process, and take note of each reading for comparison.

    • 4

      Insert the bore gauge several inches farther to take another measurement. The exact distance is up to you--you can take as many measurements as you like. Generally, a measurement every inch is more than enough.

    • 5

      Compare the readings to be sure they are consistent. If your bore measurements vary as much as 0.010 inches, there may be pitting or distortion which you might need to address. But usually the variations are much less.

    • 6

      Remove the bore gauge when your measurements are complete and wipe the barrel with lubricant to remove any corrosive materials.