Abstract: A firearm cartridge headspace gauge consisting of a threaded cylindrical barrel and threaded head is machined internally to precisely match the external dimensions of a given firearm cartridge. A scale is provided on the gauge barrel and mating head. When a cartridge is inserted into the barrel and the head is installed to the point at which the interior of the head contacts the cartridge, the scale may be read to determine any difference from the optimum headspace for that type and caliber of cartridge. Varous sizes of cartridge headspace gauges may be produced so as to conform precisely to the dimensions of any cartridge desired. The gauge provides a method of quickly and precisely checking the exact headspace of any cartridge, whereupon the cartridge may be resized as necessary after removal from the gauge.