I drop a bullet down the chamber of my rifle then stick my cleaning rod down the muzzle until it touches the bullet, then I mark my cleaning rod. Next I remove the bullet from the chamber, close the bolt, then stick my cleaning rod down the muzzle and mark it again. Finally I use my calipers to measure the distance between the marks I made on my cleaning rod. This gives me the MAXIMUM length. On the maximum length measurement I take off about .015 inches. That gives me clearence off the lands of the barrel.