NRA Life Member
In a milspec AR the field gauge is the standard. In almost every AR I've handled--including national match rifles with custom chambers--you can push a bolt home on a no-go gauge with some thumb pressure. Spent a day in Gene Barnett's shop a dozen or so years ago when he was barreling DMR rifles for the 101'st Airborne in Clarksville. Every single one he finish reamed would close on a no-go gauge with a little thumb pressure. Not on a field gauge. Reliability is paramount over small gains in accuracy in these rifles. Even in National Match rifles. The tiny degree of increased accuracy you theoretically can get in a tight chamber is easily offset by a loss of 10 points because of an alibi round that failed to fire/chamber during a match In competition at high levels you can't make that up....plus it screws up your mental game which is a big component in rifle matches.A (civilian) field gauge should be 1.4700. No go should be 1.4670.
The no-go give 0.003 more "allowable error" than the field (or if someone is measuring with a dirty chamber).
A chamber between 1.4640 and 1.4670 has more accuracy potential than 1.4640 to 1.4700 (or military 1.4730). The down side is a tight chamber doesnt have as much tolerance for being dirty.