Bump Gauge
Also: Headspace Comparator
A caliper tool that measures to the case shoulder datum so a reloader can set shoulder bump and control headspace precisely.
A bump gauge is an insert, used on a caliper, that contacts the case shoulder at a fixed datum diameter and reports a repeatable measurement from the case head to that point. Comparing a fired case to a resized case at the same datum shows exactly how far the sizing die has pushed the shoulder back.
That difference is the shoulder bump, and controlling it is how a handloader manages headspace for a specific chamber. A small, consistent bump lets the case grow on firing and return slightly on sizing, which supports both reliable chambering and long brass life.