Abstract: A depth gauge for measurement of effective loaded cartridge length in relation to the dimensions along the chamber of the rifle or other firearm. The gauge comprises a caliber cap having an aperture corresponding in transverse dimension to the land diameter of the rifle barrel. A guiding body is longitudinally adjustable on the caliber cap and includes inside surfaces complementary to the cartridge diameter. A loaded cartridge is aligned within the body and caliber cap with its bullet nose resting within the aperture. The outer end of the body serves as a length reference by which comparisons can be made between the effective cartridge length and the rifle chamber dimensions.