No-Go vs. Field HS guage


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Fatelvis
January 24, 2003, 09:18 PM
I know this will be easy for you guys-What is the difference between a "No-go" headspace guage, and a "Field" guage? Thanks-

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JohnBT
January 24, 2003, 11:27 PM
The short answer to your question is that if your gun will chamber the Field gauge you have a diaster on your hands - way too much headspace.

Saved from Brownell's Tech Talk:

"If the bolt closes with no feel to the handle with a GO gauge in the chamber, open the bolt, remove the GO gauge, and replace it with a NO-GO gauge. Again, gently attempt to close the bolt. The bolt handle should stop or show resistance to closing at some point before it fully closes. DO NOT force the bolt handle closed. Even if the bolt handle closes 98% before you feel resistance, the headspace is normally considered to be within specs.

If the bolt handle closes fully on the NO-GO gauge, repeat the test with a FIELD gauge. If the bolt does not close completely with this gauge, the headspace is on the long side, but the rifle can usually still be used with factory ammo, if the cases will not be reloaded and there are no other problems present. Never fire a gun that closes on a FIELD gauge. If it is fired, the chances are extremely high that you will get case ruptures"

Jim K
January 27, 2003, 02:24 AM
GO and NO-GO gauges are used at the factory when adjusting headspace by reaming the chamber on a new barrel. But headspace is not a constant. Firing will ultimately compress the bolt lugs and bolt lug seats (assuming a bolt action or similar rifle) so headspace will increase. This usually takes thousands of rounds, more than most of us will ever fire a single gun, but it does happen. The gauge used to check used rifles is the FIELD gauge.

This all must be understood in terms of ammunition manufacture to certain +/- tolerances. The GO gauge ensures that the rifle will function with the longest cartridge that is within specifications for the caliber. The NO-GO gauge ensures that the rifle will be safe with the shortest cartridge that is within specifications.

The FIELD gauge says that the rifle may be unsafe with the shortest cartridge that is within spec. It may be safe with medium or long cartridges, but there is no way (short of a cartridge gauge) to ensure that only long cartridges will be used, so the rifle that closes on a FIELD gauge should be considered unsafe.

So why the business about cartridges? When a cartridge is fired, it is usually pushed forward in the chamber by the firing pin. The primer ignites and the gas generated by the burning powder pushes outward on the walls of the cartridge case. The thin forward walls are pressed tightly against the walls of the chamber. But the thicker rear end does not expand; instead, it tries to push back on the bolt face. If it can move the bolt at all, it will move back, leaving the forward part of the case in place because it cannot move.

The case will stretch. This happens almost all the time, but if the headspace is too great, and the base of the case can push back too far, the case will literally be pulled (or rather pushed) apart, releasing high pressure gas into the action. That is why excess headspace is dangerous.

Jim

Fatelvis
January 27, 2003, 08:54 PM
Thanks guys!

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