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Old 10-03-2013, 11:28 AM
ETicket ETicket is offline
Join Date: Jun 2013
Location: Ventura, CA
Posts: 102

Originally Posted by en bloc View Post
Nice work Ericc.
This post should be made STICKY for posterity.

I agree with this, it should be a sticky. It explains the procedure very clearly.
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Old 09-10-2015, 04:53 PM
Coastal190 Coastal190 is offline
Join Date: Jun 2015
Location: St. Marys, Georgia
Posts: 9

I should have read this post before I tried to check my headspace. I thought I had a problem but after performing this procedure I know now that I don't have a problem.
A BIG Thanks!!
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Old 04-21-2017, 06:43 PM
HB of CJ HB of CJ is offline
Join Date: Nov 2011
Location: 42N -123W OR USA Kinda
Posts: 1,125
Default Incomplete Description And Why ...

Couple things to add. Rotate the gage about 1/10th circle each time pressing the bolt. You want an average of at least 10 different placements. Yes it makes a difference.

You do not have to disassemble the bolt. There is a trick to this. But you do have to remove the bolt and snap or place the gage in the bolt face. Check for engagement.

Make sure the ejector is compressed fully. Make sure the extractor rim is not obstructing the gage pressing completely flat against the bolt face. No slide or op rod with this.

Repeat the gage rotation again. You are establishing an average of 10 checks. To be sure repeat with a different manufacture gage. More better. Get an average.

Yep .... just a few pounds of pressure needed. Do not force. Like said, the bolt must lock up completely. Repeat with longer gages until you determine the head space.

Why all of this? You need to determine a measurement. With only one gage and one check, statistically it MAY not be valid. Using several different manufactures helps.

Hope this helps. Passing it forward. Respectfully.
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Old 05-31-2017, 04:11 PM
Kestrel4k Kestrel4k is offline
Join Date: Dec 2012
Location: Oregon
Posts: 1,590

I was trying to put '06 headspace gauge measurements in order; came across this thread in a search so figured I'd ask here.

From perusing the various 'Navy' threads in past years here, I put together the following specs to help me keep track of .308 & 7.62NATO:

1.630": .308 Go
1.634": .308 No-Go
1.6355": 7.62 Go
1.6375": 7.62 No-Go
1.638": .308 Field
1.640": .308 Max
1.6455": 7.62 Field

I would like to put together the same listing for '06; I'm thinking it should be simpler as the civilian & military spec seems to be the same ?

A brief search of Forster gauges didn't turn up the '06 measurement specs, and was hoping someone here could point me in the right direction for a definitive '06 listing comparable to the one above?
So far, for .30-06 I have:

2.049": Go
2.055": No Go
2.058": Field

Sounds right?

Last edited by Kestrel4k; 05-31-2017 at 04:19 PM.
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Old 06-01-2017, 01:21 PM
Unclenick Unclenick is offline
Join Date: Nov 2010
Posts: 987

Both .30-06 and .308 have case dimensions identical to their military counterparts. When you examine the SAAMI standard you find a little bit of shoulder and chamber interference is allowed (a small overlap in minimum chamber and maximum case) for the ability of a rifle bolt to compress and upset the case diameter outward a little. As a result, rifle operation is not certain to be impeded until the case body has expanded to fill the minimum chamber diameter.

In the 7.62 gauge numbers you found, that SAAMI overlap is eliminated. I surmise it is because the extra chambering effort from a maximum length cartridge getting into a minimum size chamber would increase the chance of a fixed-firing pin gun design igniting primers before the action is fully locked, increase the chance of a slamfire in a floating firing pin design, and because the extra effort increases the likelihood of a jam even if firing is not affected. Again, that is surmise on my part, but it makes sense to me.

The .30-06 is older and dates to when headspace was taken from the breech to the shoulder and case body intersect, which was used as the reference diameter back then. In .30-06 that diameter is 0.4425", and Hatcher gives the headspace to that point from the breech as 1.940". SAAMI calls that dimension as 1.9399", so they are within a tenth of a thousandth of one another and for all practical purposes are the same number.

From the SAAMI drawing, the minimum chamber is 2.0487", which would be the GO gauge carried to one more decimal place than you did. The maximum they give is 2.0587", or 0.010" bigger than minimum. That's the same minimum to maximum difference the standard has for .308 Win. On page 214 (page 226 in Acrobat Reader's page counter) of the 2015 standard I linked to, they give those two dimensions as minimums and maximums for gauges.

You'll have noted with the .308 that the FIELD REJECT gauges are -0.002" shorter than the SAAMI maximum. I believe that's an industry consensus number that has developed among the gauge makers over time, as it does not appear in the SAAMI standard. I don't know where it originated. The .308 Win NO-GO gauge is simply 0.004" shorter than FIELD REJECT length. The NO-GO applies only to a newly cut chamber's maximum, thereby reserving at least 0.004" of space for a little lug setback and other size settling to occur as the gun is used.

For .30-06, that -0.002" adjustment to maximum number is not employed, and the FIELD REJECT number matches the SAAMI maximum. The NO GO is, again, 0.004" shorter than FIELD REJECT, so the same growth allowance is used.

Based on the above, my expectation would be the nominal gauge numbers would come out as:

2.0487+0.0002" GO (2.0487"—2.0489")
2.0547-0.0002" NO-GO (2.0545"—2.0547")
2.0587-0.0002" FIELD REJECT (2.0585"—2.0587")

I suggest you call Manson Reamers or someone else actually making the gauges to ask how the particular allowances and .308 Win numbers evolved.

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Old 06-01-2017, 01:51 PM
Kestrel4k Kestrel4k is offline
Join Date: Dec 2012
Location: Oregon
Posts: 1,590

Thank you for the very good explanation, UN.
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field, gauge, headspacing, no-go

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