Its a depth gauge of sorts to determine cartridge head clearance. You seat a primer about 1/2 the way, chamber the case then lock the action to battery which will seat the primer deeper into the pocket when the shoulder, rim, belt and etc bottoms in the chamber. The amount the primer protrudes above the case head is the cast head clearance. If the primer protrudes .002" the case has .002" head clearance.
Bill
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