On an AR a field gauge is enough. If the headspace is off the whole barrel assembly or the bolt is trash, because you cannot adjust it. The bolts are all supposed to be made to the same dimensions. So, you could hope you got a bad bolt.
If a new bolt won't headspace, then the barrel is trash. The barrel extension has to be headspaced at the time the barrel assembly is put together. Once it the barrel extension is set at the proper headspace the extension is drilled and pinned with the indexing pin that lines the barrel up in the upper receiver. Then the gas port is drilled to be at top dead center based on the index pin. If you had to adjust the barrel extension after the fact, your index pin or gas port or both would not be lined up properly to allow sights to line up, or the gas block and gas tube to align.
So, generally a new barrel and new bolt are going to headspace, but this should still be verified.
Last edited by SSGN_Doc; 12-31-2013 at 05:24 AM.