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Sinclair Bump Gage Inserts are essential in determining the amount of shoulder bump when setting up full length sizing dies. These inserts are designed for use in our comparator bodies and are tailored to match the shoulder angles of specific cartridges. Rather than measuring to the datum line, which can be difficult, our Bump Gages indicate the shoulder of the case, providing a much more easily achievable measurement of how much your cases are being sized. For bolt action rifles, we recommend a bump between .001" to .002", while AR-15 type rifles are best with a bump between .003" and .005". Apart from these, the Sinclair Bump Gage is also useful in separating brass and loaded ammunition if you have numerous rifles chambered for the same cartridge. Lastly, it's also a way to check your loaded ammunition in such cases.