The gauge of a shotgun is calculated by using lead balls that are the same diameter as the shotgun barrel. The number of these lead balls it takes to equal 1 pound is equivalent to the gauge of the shotgun. The items needed to measure shotgun gauge are rubber gloves, shotgun, pen and paper, scale, a precise measuring tool, and up to 20 lead balls the same diameter as the shotgun barrel. This process only takes a few minutes to complete.
- Check the shotgun for safety before beginning
Make sure the gun is unloaded. Never point the gun at anything you don't want to destroy. Always keep your finger off the trigger while working on the gun.
- Measure the diameter of the barrel
Using a precise measuring tool, such as a set of calipers, accurately measure the bore diameter of the shotgun barrel. Record this measurement on a piece of paper for later use.
- Determine which size lead balls to use
Put on the rubber gloves to avoid contact with the lead balls. Using the calipers again, measure the lead balls to find the balls that are the same size as the bore measurement recorded earlier, and set these aside. It may take up to 20 lead balls to complete this measurement, but larger-bore shotguns will require fewer lead balls.
- Weigh the balls to find the gauge
Use the balls that were set aside in the previous step to find the gauge. Start adding them to the scale, one at a time, until they add up to 1 pound. The number of balls it takes to add up to 1 pound equals the gauge of the shotgun. For example, 12 balls of 12-gauge diameter weigh 1 pound, and 10 balls of 10-gauge diameter weigh 1 pound.