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You’ll be touching the trigger, so it’s imperative that your rifle is unloaded for these tests. Remove all ammunition and clear the chamber before beginning. Use a tape measure to find the distance between the center of the trigger and the center of the back of the butt plate or recoil pad. This is the LOP of the rifle. In case the LOP turns out to be too long or short, you’ll need to know the measurement to determine if your stock should be trimmed or replaced. Most factory rifles have an LOP between 13.00 and 13.75 inches (33.0 and 34.9 cm).

Summary:
Begin with an unloaded firearm. Measure the distance between the trigger and the butt plate or recoil pad. Write down the measurement.