I've used an OAL gauge before to measure my bolt face > lands and then subtracted .010". I ended up with a very accurate load that would only single feed because it was longer than my magazine would allow. In that rifle, I switched from Nosler BT to Sierra BTHP and gained some space from not having the plastic tip on the bullet. ie: the ogive was closer to the lands than the Nosler BT when the OAL was shortened to fit the mag.

I'm not sure how a comparator works or what is to be gained from it's use. Can someone explain it so I can understand?