Out of the millions of civilian AR-15s and the millions of military M-16s there are only a hand full of accounts of "supposed" headspace issues. Unless you have several thousand rounds on that BCG you are nowhere close to having a headspace issue. I garantee your bolt lugs will break before you ever get to that point! In the military bolts get swapped all of the time during batch cleaning. I've never seen a M-16 upper or bolt with matching numbers. If it were an issue they would be marked like a bolt action rifle.han
What part of tight tolerance and total interchangiblity are we missing. The only issue arises with a chamber being improperly reamed (which is quite rare). That is what the headspace gage is for, not the bolt. Never heard of a Colt chamber being out of spec from the factory. After millions of uppers produced you'd think they'd have it right. Ever hear of a bad Colt, Spikes Tactical, Sabre, BCM or FNMI?