Head spacing with only a FIELD gauge.

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Originally Posted by tenOC

They're actually stamped 1.290, 1.296 and 1.299.

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Mine are the same, I should have checked before posting - they weren't buried as deep in my Carbine "stuff" as I thought they were! - Bob

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Originally Posted by Sunray

Headspace gauges do not 'measure' anything. They only tell you if the thing is within tolerance or not.

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:confused: :rolleyes:

A Head Space Guage is used for measurement.

1.290"
1.295"
1.300"
Above are the USGI examples I used from TM9-1276_1947
Head space is MEASURED with a set of GO,NO-GO and FIELD guages.

Maybe a definition of Head Space (In Firearms) may be required here:

Head Space is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt.

These Head Space Guages were required by US armorers to be sent yearly in to a Arsenal to be Inspected. Above from TM9-1276_1947 page 18, in section 15 part 4 under Note.
Must have been for Inspection and Calibration of Length and Diameter.

You still think they were used for 'Close Enough' type calculations. Like your only a 1/3 of a quart of oil low ?

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Originally Posted by tenOC

So....there ya go. China is always right, aren't they?

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This is an oldie:

The fact that an instrument has a resolution of 0.001" does NOT mean that it is accurate to 0.001".

The slightest bit of grease or dust on the faces of a gauge will affect the reading. As will the temperature. Or the slightest skew of the object being measured. If you are really concerned about accuracy, then you need a reference bar. In high-quality micrometer cases there used to be an hook spanner and a 1" reference bar for adjustment. I am lucky enough to possess a cased 2" bench micrometer that reads to 0.0001". The reference bars have plastic grips for a very good reason - with the bench micrometer you start to detect the difference between a "cold" measurement - i.e. out of the cupboard - and "hot" - i.e. after you have held the object in your hands for a while.

So while your digital gauge may be readable to a resolution of .001", do not trust it blindly. At best it is indicating that a measurement of, say, 1.296" is closer to 1.296" than 1.295" or 1.297" provided that all parts are spotlessly clean and at the same moderate temperature.

Yeah. :rolleyes: Thanks for teaching me about grease and skewing an object.

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Originally Posted by tenOC

I have one that closes easily on the NO-GO. But doesn't on the FIELD.

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From a Brownells how to:

On older military rifles:
An old military surplus rifle like a Mauser, Mosin Nagant, '03 Springfield, Enfield, Arisaka, and others may close on a NO-GO gauge. If it does, check it with a FIELD gauge. A FIELD gauge measures the absolute maximum allowable, safe headspace. If the bolt does not close on a FIELD gauge, and the gun is in good condition, it is generally safe to shoot.

To add:
However, the strength of these old guns can vary a lot depending on when and where they were manufactured. Headspace is not the only factor in deciding whether one of these guns - or any gun for that matter - is safe to shoot. When in doubt, take it to a qualified gunsmith to have it checked out.

Link:
https://www.brownells.com/guntech/he....htm?lid=17125

Best to have a real clean chamber, dry with no oil or solvents.

:thup:

I feel a lot better reading about the 1.302 value being the lowered bar.

Here at work we do services on military rifles (M4A1s) and MGs (M249s and M240s) week in and week out. We only use the "Field" gage.

When I am at home, I use both the Go and No-Go gages. If it fails No-Go I don't see any reason to take a chance on shooting it.

Al

TM 9-1276 February 1953, page 14
1.290
1.295
1.298
1.302
Page 37 for usage of each gauge.