Maybe someone can explain this in simple terms?

[mugsie]

I have built several bolt rifles and can understand head spacing. I move the barrel in so that the bolt closes on a gage and basically that sets the head space. With an AR, I understand they don't need to be headspaced, that somehow they headspace off the bolt? How does this work? If I build an AR I can just screw a barrel on and not worry?
Thanks....

 

[303tom]

Because on AR`s the (Bolt Carrier Group) moves, it is not solid as on a Bolt Action`s.

 

[sugarmaker]

AR's headspace like any other bottleneck rifle, but the locking lugs are machined into a screw on barrel extension, which the bolt is then fitted to.