There are several brands of headspace guages available.
Bushmaster sells a set of GO, NO GO, and FIELD
Brownells sells Clymer, Manson, and Forster. Only have GO and NO GO for Clymer and Manson. Forster is the complete set.
Brownells also sells a guage that is "made to Colt factory's special dimensions for M16/AR-15 rifles. It measures the maximum factory acceptable headspace. Is this a field guage?
Any brands to stay away from?