Check headspace before installing the barrel.
Strip the bolt.
Clean the gauge, bolt, barrel, barrel extension, and locking lugs.
Insert the gauge, and see if the bolt will lock. (Free-spinning in the barrel extension is fine when using a "go" gauge.)
I've built half a dozen, or more.
Most had no issues with headspace (and weren't checked with actual gauges - at least initially).
But the one that did have bad headspace... I could have saved myself innumerable headaches by checking headspace before installing the barrel, rather than after running into chambering issues.