Case Gauge
Also: Cartridge Gauge
A precision gauge that checks a sized case or loaded round against minimum chamber dimensions to confirm it will chamber reliably.
A case gauge is a machined steel block cut to minimum chamber dimensions that lets a handloader confirm a resized case or finished round falls within spec without the rifle present. You drop the case in, and the relationship between the case head and the gauge’s machined steps tells you whether the headspace and overall length are correct, so a round that fits the gauge should feed and close in any in-spec chamber.
The gauge is most useful right after full-length sizing, where it shows whether your shoulder bump pushed the case shoulder back the intended amount. If a case sits proud of the gauge it needs more sizing, and if it drops too deep the shoulder has been set back too far, both of which the gauge catches before the round ever reaches the range.