My experience has been that AR bolts and barrel extensions are usually held to very tight tolerance for concentricity and specified dimensions. Most makers know that users may take a bolt and use it on multiple uppers without checking headspacing so they try and avoid any issues by holding as tightly as possible to acceptable consistent standards.
That's just my anecdotal experience from the many bolt, carrier croups, and barrels I've handled so others' experiences may vary.
The greatest variation in BCG quality usually has more to do with quality of materials. Many companies have, from time to time, cut costs by using substandard material and coatings on the bolt, gas key, carrier, springs, and extractors.
Depending on what phase BM was in when it made your BCG I doubt youll have any trouble with accuracy, and bet you're more likely to need to replace an extractor sometime down the road. Without looking at your bolt personally I couldn't make a judgment on that either since there were times when BM made everything just fine.
Edited by Wildhorse (12/03/15 11:57 AM)