I didn't see anybody suggest a case check gage.
A case check gage is used to ensure the cases are properly sized and that the bullet is seated sufficiently deep as to (probably) feed and chamber correctly.
They make a great reference point to ensure the ammo will fit the chamber without having to shoot to prove it.
One can also use the disassembled barrel as a check gage but this only proves the ammo will fit that barrel. Another barrel might have a tighter chamber.