Headspacing, Bolt Matching and Barrel Extension Questions

[MistWolf]

How is the headspace adjusted in an AR type rifle? If I needed to change the bolt and the headspace was too tight or too long, how is it brought into spec? How is a bolt matched to a barrel?

Is the barrel extension pressed onto a barrel or what? Or is the barrel extension and the barrel a single piece?

I'm gathering parts for a build and I'm unsure about these aspects of the AR.

Thanks

[constructor]

Extensions are screwed on to the barrel and the gas port aligned with the extension. If the bolt and barrel extension combination do not hit between the 'Go" and "No-go" gauges you drop the bolt and reach in the bin for another. If they don't match it could get a little expensive for the consumer sending bolts back and forth. Most work within spec. but, that is why some companies sell only bolt and barrel combos.