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Sinclair Bump Gage Inserts are specifically designed to measure the amount of shoulder bump for setting up full length sizing dies. Our Bump Gages precisely fit into either of our comparator bodies and are machined to fit the particular shoulder angles of the cartridges. Rather than measuring to the datum line, these gages give a more assessable measurement as they indicate off of the shoulder of the case. This makes it very easy to determine how much you are actually sizing your cases. For bolt action rifles, we typically recommend a bump between .001" to .002", while for AR-15 type rifles, a bump between .003" and .005" usually works the best. This tool can also be quite useful in separating your brass and loaded ammunition if you happen to have multiple rifles chambered for the same cartridge. In short, Sinclair Bump Gage Inserts are an essential tool for checking your loaded ammunition and ensuring that your brass is always in impeccable condition.