Why a no-go gauge?

Field gauge is absolute max. A no go gauge is useful for checking if something has changed. If the firearm did not close on a no go ,and it does now, you for sure have a problem. In reality if it closes on a no go gauge some investigating needs to be done.

If a bolt doesn't close on a go gauge, the headspace is too short and it won't necessarily close on SAAMI spec'd ammunition.
If a bolt closes on a field gauge, it has excessive headspace and will either prematurely wear out brass or cause potentially catastrophic case head separation on an otherwise properly loaded cartridge.
If it closes on a NO-GO gauge, the next step is to see if it closes on a field gauge.

0. If it doesn't close on a GO, it's too short and should be reamed.

1.If it will close on a GO, and not close on the NO-GO or field, it's a good chamber

2. If it closes on the GO and NO-GO and not on the field, you have a max safe length chamber.

3. If it closes on the GO, NO-GO and field, don't shoot with that gun, the chamber is too long.

rsrocket, thanks for putting it so clearly. Remembering these logical relationships, for some reason, is always difficult for me (perhaps because I don't gauge chambers very often). I'm going to copy/paste your comment into a document where I "store" miscellaneous reference items.

So a no go gauge is a "might have problems please reconsider gauge" and a field gauge is a "stop I really mean it" gauge?

IMO, a no go gauge is really for a rebarrelling or reheadspacing or some other gunsmith job where you would want your chamber to be at a more minimum spec and then be allowed to wear from there with use. Or for a brand new factory produced gun.

I don't want my brand new Kimber rifle to have a chamber that closes on a go gauge and a no go gauge but not a field gauge.

Most any gunsmith would not allow a customers gun out the door that chambers a no go gauge.....I would hope....and neither should anyone else.

OrangeCat said:

So a no go gauge is a "might have problems please reconsider gauge" and a field gauge is a "stop I really mean it" gauge?

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Interesting way of putting it,but, essentially correct. Depending on the firearm I wouldn't fire it if it will take a no go gauge. AT that point the chamber is out of spec. Some older rifles , and military arms, had pretty large chambers for a variety of reasons. Military to ensure oversize or dirty ammo would chamber. The field gauge made sure it was still safe to shoot with ammo that was provided for the arm.

earlthegoat2 said:

a no go gauge is really for a rebarrelling or reheadspacing or some other gunsmith job where you would want your chamber to be at a more minimum spec and then be allowed to wear from there with use.

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But the chamber doesn’t wear from use, the throat does, and headspace has nothing to do with that.

Or am I misreading what you are saying?

Here are links to a 2 part article from Brownell’s that explains the use of headspace gauges very clearly.

https://www.brownells.com/aspx/learn/learndetail.aspx?lid=12555

https://www.brownells.com/guntech/headspace-gauges-and-how-to-use-them-part-ii/detail.htm?lid=10389

Nature Boy said:

But the chamber doesn’t wear from use, the throat does, and headspace has nothing to do with that.

Or am I misreading what you are saying?

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Normal wear and tear on bolts, locking lugs and mechanisms can increase headspace over time. A field gauge will tell you when it becomes excessive.

higgite said:

Normal wear and tear on bolts, locking lugs and mechanisms can increase headspace over time. A field gauge will tell you when it becomes excessive.

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This is what I was referring to regarding wear.