Sinclair Bump Gage Inserts are used to measure the amount of shoulder bump when setting up full length sizing dies. Our Bump Gages install in either of our comparator bodies and are machined to fit particular shoulder angles of cartridges. Rather than measuring to the datum line, these gages indicate off of the shoulder of the case to get a better, more easily assessable measurement of how much you are actually sizing your cases. We usually recommend between .001" to .002" bump for bolt action rifles and between .003" and .005" for AR15 type rifles. Also a useful tool for keeping your brass and loaded ammunition separated if you have multiple rifles chambered for the same cartridge and checking loaded ammunition.
Big Book Catalog, Issue:66, Page:513
Sinclair, Issue:3B, Page:041