I see a lot of people using Field gages.Quote:
Factory Specified Size Measures M16/AR-15 Maximum Headspace Made to the Colt factory's special dimensions for M16/AR-15 rifles. It measures the maximum factory acceptable headspace; an essential check on guns that have seen extensive service or have been rebarreled.
From what I understand, they were used by military armorers in a time when they needed every available rifle that could shoot. For maximum accuracy, I use only the "go" gage as a field gage is way too long.