OK, new at this and was curious on what it would tell me: Took a once-fired brass (shot out of my rifle), put a Sierra Matchking bullet in the brass (long), colored it with a magic marker and chambered it. Chambered fine, total length is visibily shorter and the marker is rubbed off in a little straight line almost but not completely around the bullet. What information do I have in front of me? Anything usable or do I still need an OAL tool? Does the brass give me any reliable information regarding the proper distance for the neck and shoulder settings for my dies? Anything else? If it helps, caliber is 30-06. Thanks for your help with this.