Thanks for the reply. I realize headspacing on the AR series is mainly with the barrel assembly, but does the bolt come into it at all? I know that it is not wise to use a used bolt in a new barrel, but is there any reason that the two (receiver and bolt) should be ordered together. I have a new quality bolt and am interested in a Bushmaster upper receiver assembly, and would like to use my bolt if possible. From everything I've read, a new bolt and barrel should still be checked for "headspacing" together, is this not true (as long as it's a new quality bolt)?