To bore-sight a scope-mounted 12-gauge shotgun barrel, remove the primers from a spent 12-gauge shell and a spent 20-gauge shell. Put the open 12-gauge shell in the chamber and insert the open 20-gauge shell in the muzzle. Secure the barrel in a vise or sandbags. Sighting through the barrel, line up the primer holes on your target. Now, simply adjust the scope to put the crosshairs on the same spot. – Thomas Stormanski, Chester, VA