I purchased a criterion barrel and on their site and many other sites that sell barrels there is the disclaimer that the purchaser should have a gunsmith check for proper spacing and installation. In most industries with swappable parts, they usually come ready to install with minimal work involved besides piecing together. Most other parts on the rifle also seem ready to attach as well. Do you check head spacing when you purchase a new one? Should I? Or is this simply a safety notification to cover the seller? I don't want to be negligent when putting it together but I'm also interested how ready the barrel.

It's never a bad idea to check the headspace. However, on the AR platform, with quality components, it's rarely ever necessary, nor does it develop into an issue.

So, if you have a quick and inexpensive and reasonable way to get it checked, do so - but don't sweat it if you can't.

My $.02

No.  I question its validity even with used parts.  Primers will probably pop first.  Ever read a story of an AR KB due to headspace?  Me neither.

AR parts are typically advertised as mil-spec.  If they truly are mil-spec, along with modern manufacturing techniques, the vast majority of the time AR parts are probably interchangeable in most manners from a safety standpoint.  I check my head space on all ARs and have never found a barrel/bolt combination to be out of spec.  It is simple and makes my anal retentive self feel better.  I was going to provide a Brownells link specifically of what I have but am not able to find mine in my tools; darn...  Mine are field gauges for 223 and 556.

However,  Barrels are a different matter.  All of my sub $200 AR 556 marked barrels measure closer to .223 and I have reamed them all out to 556 specs.  The old Bushmaster barrel was by far the worst and I had to remove more metal than expected.  My > $200 BCM CHF 556 barrel measures to 556 specs, but just barely, and is on the tight side.

I would say yes.

The last two barrels that I received from Ballistic Avantage failed the Go-gauge with 4 different bolts (two LMT Enhanced bolts and two Colt bolts).

May not KB, but if you get a short chambered barrel you will put undo stress on it and shorten barrel or bolt life.  I might want to send that barrel back after testing with few different bolts to find out it is short.

I have found 4 short chambered barrels in the last couple years, checked with multiple bolts. The bolts all passed with other barrels.

I feel that, with the huge number of "new" barrel and bolt makers out there (often with unknown pedigrees), it's important to check headspace on a new build - with the bolt you'll use in that build.

If I bought both the bolt and barrel from BCM, for example, I'd still check headspace.  BCM is not an actual manufacturer; they have stuff made for them.  (GASP!!!)   So "in spec and per customer requirements" can still mean tolerances stack in the wrong direction.  I trust BCM to stand behind their stuff, but I also know Murphy's Law applies here.

If you build more than a couple of ARs, spend about $65 and buy Forster GO and NO GO gauges (I got mine from Brownells).  Learn how to use them properly.  Maybe they'll never tell you there's a problem.  But the only way to make sure there's no problem is to check.

It is insurance.... for your face.    You are trusting parts put together from various vendors and the result of a mistake can be a gun exploding in your face.

I am in the process of building my first AR. After lots of research, I purchased a set of Go/NoGo gauges. Most of the stuff I read and videos I watched recommended checking headspace. I could not find, though I assume, that AR manufacturers such as Colt, S&W, Ruger, etc. check headspace on every AR. If I remember correctly, I saw a video on YouTube that had a guy from Faxon, that recommended checking headspace. I see no reason I shouldn't either.

Thanks! This is video I was referring to. I believe it was also a Brownells FaceBook live presentation.

https://youtu.be/bRxTQjil4Bc

You want to build? Do the job right and check it the right way. Take the extractor out and check it with the right tool for the job. Be a craftsman not a hack.

xccoaster I'm envious. Criterion barrels are really sweet. I have ordered them for my bolt guns and all were superb shooters. Would love to try one for my AR, hope you will share your results once you start shooting it. As far as head space it's never a bad idea. If you don't want to buy the gages and your a reloader just remember - base to datum- no go is plus .004  Head space is not hard to check at all. Every new bolt carrier group should always be stripped 100% anyway.