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The Sinclair Bump Gage Inserts are utilized in measuring the shoulder bump during the full length sizing die setup. To accommodate various shoulder angles of cartridges, these gages fit into any of our comparator bodies and are precisely machined. Rather than measuring to the datum line, these gages determine off of the case's shoulder a more straightforward and readily available measurement for how much you are, in reality, sizing your cases. For bolt-action rifles, we usually recommend bumping between .001" to .002" while AR-15 type rifles should bump between .003" and .005". These gages also come in handy for keeping your brass and loaded ammunition separated if you have multiple rifles chambered for the same cartridge while checking loaded ammunition.