Can someone tell me what 5.56 NATO Go, No-go, and field gauges are supposed to measure?  NOT what the .223 Remington gauges are supposed to measure.

Thanks.

Help you know whether or not your rifle is within acceptable headspace specs and that the B/C installed is compatible (in spec) with upper it's installed in. If I purchase an upper at gunshow and it did not come w/ a B/C installed from factory I always make dealer do a go/no-go check before I pay my money as I want a dedicated B/C for every upper I own (personal preference). That said there are some here that use the same B/C for several uppers, but suspect that they know via guages that their in spec.

Generally speaking a new B/C and a new upper SHOULD be a compatible match, guaging provides the peace of mind that indeed no headspace problems exist.

Hope this helps, my take on you question,
Mike

The answer to your question is tacked at the top of the Maintenance and Cleaning forum.
www.ar15.com/forums/topic.html?id=61558