previous posts are all pretty much correct. I've seen .223 gauges frequently close on the no-go guage without a problem, and this is on brand new barrels. It all depends on who is chambering the barrels, you can ream them deep, shallow, use a different style i.e. "Wylde" or even variances in barrel extensions, many things can effect headspace. You should try to do it by hand, with the extractor and ejector removed from the bolt, the bolt should be reasonably new. Many tech manuals say to try to close the carrier/bolt combo by ramming it home, but this is not the proper way to "feel" the headspace. If the go gauge "go's" then put into the lock position and rock up and down, you will be able to feel the .001", .002" or whatever amount of play there is in the headspace. Then try the "no-go" gauge, it it does not go...good, if it does, feel the play again, then try the field guage, if it goes, the barrel is trash. You can try a new barrel extension, but it probably will not help, time for a new barrel. If the "field gauge" does not fit, but the "no-go" does, you are still in good shape, just take the weapon out and shoot, checking with the "field" gauge every one or two thousand rounds or so. Remember .223 gauges are tighter than 5.56mm so you are still pretty safe with the no-go.