Depending on how tight your chamber is headspace can be critical with as little as .002 to .003 differece in shoulder length locking the action up. I've tried case guages and found them lacking when setting dies. I am currently using a Stoney Point headspace guage. It fits on dial calipers and measures from the case head to the datum line on the shoulder. You can set dies to your particular chamber or to factory specs. Have used this with 223, 243, 308, 7mm. mag. with great results. Only runs around $35.00 available from Dillon, Midway, and others.