NATO Minimum rifle chamber gages check to see that the rifle will accept the longest (headspace dimension) NATO ammunition cases that are currently manufactured. They also check to see that the rifle does not have excessive headspace, which could lead to dangerous pressure problems. The 5.56 NATO caliber requires a slightly longer headspace than the commercial .223 caliber, which are commonly thought to be identical. For this reason, Forster Products also offers 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 difference in both performance and safety. Two gages (minimum headspace and maximum headspace) are available per caliber.