Field gauge rifles

[turnero]

Could someone explain to me the consequences of firing a rifle/carbine that is on field gauge. How about those on "no go"? Most rifles I test are on "no go" but there are quite a few that are on "field". All these rifles/carbines are still in use and none have broken yet.

[Robb Jensen]

A FIELD gauge is just a gauge that you use to check for excessive dangerous headspace. If it closes on a FIELD DO NOT FIRE THE RIFLE!, if it closes on a FIELD try another bolt, if it still closes on the FIELD with the new bolt try another barrel with the new bolt.

NO-GO gauge is a little smaller and many ARs even when new will close on a NO-GO gauge. Ideally you want it to not close on a NO-GO, some times you won't have enough bolts until you find one that will not close on a NO-GO gauge.

A GO gauge you use to check for enough/adequate headspace so that a round will chamber and the bolt will close and lock.

[jmart]

You risk case head separation. The case head extracts but the body stays in the chamber.

[turnero]

OK, Thanks for the warning and the explanation. Now, how do I proceed to fix this condition without changing the bolt. I mean how do I fix the barrel/chamber? I got a huge lathe, I can take off the barrel extension and I got a chamber reamer. I can also tig the port hole shut and drill another one somewhere else.

I just need some sort of guidance or maybe some instructions. I dont want to junk a few barrels before getting it right.

[jmart]

Just buy another barrel. As I understand it when barrels are manufactured, the barrel extension is installed and the chamber reamed to set the headspace. The last operation performed is drilling the gas port so it's a 12:00 WRT the feedramps from the extension.

How you remove the extension, mill away the base of the barrel, recut the threads for the extension, finish ream the barrel, and reinstall the extension so that the port's still at 12:00 I have no idea. Maybe it can be done, but it just sounds hard, plus if your headspace is that off, I suspect your barrel has a number of rounds down it and the port and the throat may be pretty well worn.

If you think the barrel is still decent, try a new bolt and measure the HS. You may find a new bolt will shorten things up a bit, but failing that I'd just replace the barrel.

[turnero]

I was kinda hoping that I can save the barrels.

Anyway, does anyone know why it's called a "field" gauge?