AR-15 Head Spacing - when is it necessary?

[ez bake]

So in the course of buying new and used parts, I've read about the fact that a worn bolt might cause an incorrect head-space when closing into the barrel extension.

Here's a dumb question - where does the problem lie when this happens (what's wearing to the point of causing an issue)? Is it entirely in the Bolt? Is it somewhat in the barrel extension?

If I pick up a new bolt and put it into a used barrel, do I have a possible issue?

What if I have a used bolt and new barrel?

How many rounds is typical of a bolt becoming worn to the point of requiring a head-space check - like is <500rds typically safe, or does this vary quite a bit?

I'm speaking to decent-quality parts on up - I presume that with low-end parts, the problem could obviously be made worse.

Just curious.

 

[Fyrtwuck]

I don't know all the particulars like specific measurements, but the simplest way to check headspace is with gauges.

They have "Field", "Go", and "No Go". I think I saw some on the Brownell's catalog not long ago.

 

[KurtM]

I have two bolts that are ...gasp....shudder....DPMS bolts that are on their 2nd and 3rd barrels respectivly and both head space just fine, and did so on the new barrel I installed. You are far more likely to break a bolt than wear one out past servicable head space. But hey what do I know I just use junk parts.

 

[ez bake]

I have two bolts that are ...gasp....shudder....DPMS bolts that are on their 2nd and 3rd barrels respectivly and both head space just fine, and did so on the new barrel I installed. You are far more likely to break a bolt than wear one out past servicable head space. But hey what do I know I just use junk parts.

...and you don't put any real number of rounds through them either

Thanks - that's what I was looking for. I've honestly never heard of someone having head-space problems before, with new or used bolts - but its often talked about.

 

[flatwins]

Along this same line, I understand how bolt rifles are headspaced but really have never paid attention to how it is done on ARs. Is it a machining process or is it done via another method?