I assembled 2 more uppers, and had 3 new complete bolt carrier/bolt assemblies. I checked headspace using a set of Clymer 5.56 NATO gauges. All three bolt assemblies close on both the GO and NO GO gauges (2 close fairly easily on the NO GO, one took a bit of pressure in both uppers). None of the bolts close on the FIELD gauge.
As for the uppers, one is a flat top mated to a new Bushmaster 16" post ban barrel assembly, the other is a Colt A2 mated to a 20" Colt post ban barrel with an unknown round count.
Is it safe to fire the uppers with bolts that close on the NO GO but do not close on the FIELD gauge?