A new Bushmaster or Colt's barrel and new bolt will always be within the correct headspace tolerances, they're individually gaged at the factory. If you insist on gaging, all you need to do is check the combination with the military M16 "gage, headspace, field" to insure they're not excessive. They won't be, not new barrels from these two sources, but you should still check. these are, after all, built by humans. You can do this before even assembling the barrel into the receiver.
There's only the one (1) gage used in the Army when rebarrelling rifles: the field gage.
Commercial ".223" gages are almost all well undersized. Some commercial FIELD/REJECT gages are smaller than the spec for a M16 NO-GO gage, meaning the rifle will be in perfect M16 condition and yes show as a reject. Using the wrong gage tells you nothing.
Based on gaging dozens of Bushmaster and hundreds of Colt's barrels and bolts I'd not hesitate a second in putting either of these new bolt in combination with a new barrel and just shooting the puppy. They'll gage even if you don't check 'em.
I don't have this confidence on other barrels or bolts.
-- Chuck