Shotguns can be used for sport, target practice, hunting, home defense or police tactical applications. A properly fitted shotgun is essential for accurate shooting. The length-of-pull (LOP) is the distance from the back of the buttstock to the trigger's front surface. Most off-the-shelf shotguns come with a LOP between 14 inches to 14-3/4 inches. This may be fine for most users, but a custom stock fitted to a particular shooter's LOP will make mounting and pointing the shotgun easier, as well as making it more comfortable to shoot.
Stand with your arms at your sides.
Bend your shooting arm at the elbow and hold it at a 90-degree angle with your palm facing up. At this point, your arm should be parallel to the ground, palm open and fingers pointing forward.
Have your assistant use the tape measure to measure the distance from the inside crook of your elbow to the first (outer-most) joint on your shooting finger. Even though this measurement is between your finger and your elbow, as opposed to your finger and your shoulder where the buttstock will mount, it is an accurate measurement of your true LOP.
Subtract 1/4 inch to 1/2 inch from the LOP measurement if you plan to use the shotgun while wearing heavy clothes or a winter jacket.
Always unload any firearm when testing it for fit, especially when testing length-of-pull on actual firearms where your finger will be on the trigger.