Ok, I am prolly gonna get flamed... but I'll give my opinion anyway:
Quoted: "Basically, unless you are installing new barrels, you don't need a "go", and even then, a factory cartridge can provide that (some would flame me on that one!)... "
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This is correct. If the bolt will close on a factory cartridge... you dont have excessivley short headspace. That is what a go gauge checks for.
no go is nice to verify the condition of the firearm (hey, it passed a no go!)... field is likely the more useful for many surplus arms. I have all three for several calibers, and I have never needed or used the "go", but often use the field and no go...
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Exactly. On a SAAMI spec rifle - a NO-GO is used to check if the chamber has excessive headspace. The bolt should NOT close on this gauge.
HOWEVER - AR-15's are NATO spec (or now Wylde, or a handful of other variations) chambers, not SAAMI. Military rifles are designed with a larger chamber spec (typically longer throat - to allow shooting higher pressure ammunition)
Therefore, it is common for a an new, in spec AR15 bolt to be able to close on a NO-GO gauge. Therefore - using a NO-GO gauge doesnt really tell you a lot. It might close, it might not.
The FIELD gauge is dimensionally closer to the max measurement we are looking for. If the bolt closes on a FIELD gauge, it is general accepted the rifle has excessive headspace, cause by a worn chamber, worn bolt, out of spec chamber, out of spec bolt, or improperly installed barrel extension. That being said - some rifles might even close and lock on a FIELD gauge, and that would not be a big deal to me.
The worst situation possible with the AR15 rifle is a tight chamber with high pressure ammunition... which is why I check to see if the bolt will lock with a factory cartridge. If it does, without having to slam it in the chamber, then I am good to go. The headspace is set at the factory by the smith who installed the barrel extension. It cannot be adjusted by you.... so really in checking headspace, all you are checking is did the manufacturer set it correctly. I have never heard of one being shipped out of spec.... though I suppose it is possible.
Again...how do you know when the bolt "closes" on an AR? I assume you remove the bolt from the bolt carrier? Is it a MUST that you remove the ejector / extractor?
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You dont HAVE to remove the ejector or exctractor. Here is what I do.
Break open rifle at the pivot pin. Remove BCA and CH. Remove the firing pin from the BCA. Drop a round in the chamber. Insert the BCA, and with two fingers on the back of the carrier, push toward the chamber. It should snap around the extractor and the bolt should lock.
So understand if the bolt is locked or not - you really need to understand how the AR15 system works. Watch the bolt as you push the carrier. Look at the lugs on the barrel extension. The bolt, because of the cam pin in the BCA, will rotate as the carrier comes forward the last few millimeters. It "locks" when the carrier is fully inserted - and flush with the back of the upper receiver. You can see the bolt lugs have rotated and locked behind the extension lugs. This supports the bolt face as the back of the chamber, for firing.
For these reasons.... having a GO, NOGO, and FIELD set is never a BAD idea. If you were a true AR15 smith, they would be required. But for the average home builder, they arent necessary.
But I think you can see why some say "just get the FIELD gauge." Personally, I think if you are so obsessed with headspace in the AR system, you really should get all three... so you have some idea of exactly where your chamber sits.