Drop a round like you shoot into the chamber, if you re-load use a re-sized round from your dies, if you shoot factory then use a factory round. Pull down the ejector so that it does not engage the round and the round falls into the chamber without the extractor under it. Do this with the barrel not mounted on the receiver. Now take a dial indicator and using the depth feature measure from the rear of the breach to the rear of the cartridge, should be between .000 and .005 inch. My .223 is .002. Larry