The Forster NATO Maximum rifle chamber gage checks to see that the rifle does not have excessive headspace, which could lead to dangerous pressure problems. 1.6455" length, plus zero, minus .0003". There can be great differences between the chambers of military rifles based on the type of action (FN, Garand, Ishapur, etc.) and also based on the amount of use the rifle has seen. Forster Products NATO Minimum rifle chamber gages check to see that the rifle will accept the longest (headspace dimension) NATO ammunition cases that are currently manufactured. Similarly, the 5.56 NATO caliber requires a slightly longer headspace than the commercial .223 caliber, which are commonly thought to be identical. For that reason, Forster Products is also offering 5.56 NATO Headspace Gages. For any gunsmith or armorer working with military firearms in these two calibers, these NATO Headspace Gages can make a huge difference in both performance and safety. Note that the 7.62 NATO Headspace Gages do not conform to SAAMI tolerances. Two gages, for minimum and maximum headspace, are available per caliber.