I know theres alot of talk on these sites about setting COL. This is just a little tool I use to help me with setting up my dies for various bullets used. You first need to establish your chambers OAL with your bullet of choice. I use a Stoney Point guage for this. I write that down and then take that exact same bullet that I used to measure the chamber and use it to set up my dies. I do not load this bullet! It goes on the shelf and is designated for this purupose. I do this with each brand of bullet I am going to use for a given rifle. Then I take this dummy case I made which I threaded for a 1/4 20 and set up my seater die. I neck size the case and seat a bullet and check the measurement. You simply thread the bolt in until it shoves the bullet out of the case, readjust die and repeat until you have the die set to your measurement. I know its not rocket science and will probably help out the new guys more so than the seasoned guys buts it what I do.