Diagram of .308W & 7.62mm headspace gauge datums?

[Thunderbox]

Without wishing to spark another huge long .308/7.62 thread, I have a request for factual information:

Does anyone have a good drawing/diagram of the way .308W and 7.62mm gauges relate to the respective chambers?

I was specifically after a drawing of the actual datum points - i.e. middle of shoulder, front of shoulder, etc - for a standard weapon such as a rifle.

Would also like to see an EMER drawing or similar of any gauge that happens to be specific to a weapon system, if it differs from the rifle datums.

(I know that there are different 7.62mm gauge limits for different weapons, but is this due to different datum points (maybe necessitated due to chamber wear patterns) or different headspace specifications (i.e. bolt body positioning/rebound in an MG)?).




Might be a good idea for a sticky? I.e. no discussion/ no debates, but a list of diagrams, specifications and military instructions only - which we can use as a factual source data.

Maybe if someone is an ace at AutoCad we could produce a set of master reference drawings?

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[Peter Laidler]

Phew............ Difficult one Thunderbox. The BEST thing you could do is to back engineer the drawings from a known/calibrated gauge. Let's say the gauge was 1.643 for example, then you'd need to work forwards 1.643 from the base to identify that EXCACT diameter on the neck to identify the exact point at which the calibration commences at. The drawings for the gauges weren't in the eMER - or anywhere that I can recall although the INspectorate might have them. Alas, now all buy-off-the-shelf commercialised!

As a little aside. to actually calibrate the gauges, we used to drop the gauge into a dummy 7.62mm 'chamber'. Let's say that the depth of the chamber was 1.5" from that datum diameter/line/point to the rear end of the dummy 'chamber, then a DTI would measure exactly the amount of the rear of the gauge protruding out of the back. Let us say that .143" was pritruding at the rear, then you can add 1.5 to .143 and there's the 1.643" gauge. That's it......... Reject gauge for the L1A1 rifle.

I have access to the workshop calibration block if you have some unmarked or unknown gauges but don't tell everyone!

[enscien]

UK military headspace dimensions for 7.62mm NATO chambers and cartridges are measured to a datum diameter of 0.4000 inches, nominally mid-shoulder.

[Bruce_in_Oz]

Be careful with "NO_GO" gauges.

These often seem to be "weapon-specific", especially when you start working with machine-guns. I cannot remember offhand the exact figures, but the "official" NO_GO for 5.56 rifles (AUG, M-16A2 etc) are different and NOT the same as used on the Minimi LMG.

I know there was a difference between the NO_GO gauges for the L1A1 and the Parker-Hale M-82, and I understand that there is a different gauge again for the AI rifles. The NO_GO for the M-60 was different again.

SAAMI gauges are for "sporting" rifles and handguns. Belt-fed toys are a very different matter.

I will go for a dive in the crypt.

[Peter Laidler]

Every 7.62mm weapon in UK service had a different gauge spec. And then there were the usual unit and Field/Base specs too!

[Son]

---------- Post added at 08:38 PM ---------- Previous post was at 08:32 PM ----------

[/COLOR]

Be careful with "NO_GO" gauges.

These often seem to be "weapon-specific", especially when you start working with machine-guns. I cannot remember offhand the exact figures, but the "official" NO_GO for 5.56 rifles (AUG, M-16A2 etc) are different and NOT the same as used on the Minimi LMG.

I know there was a difference between the NO_GO gauges for the L1A1 and the Parker-Hale M-82, and I understand that there is a different gauge again for the AI rifles. The NO_GO for the M-60 was different again.

SAAMI gauges are for "sporting" rifles and handguns. Belt-fed toys are a very different matter.

I will go for a dive in the crypt.

I wrote down the sizes and uses of all the gauges I have for 7.62 in my own collection at home and my box at work.. IIRC there were 9 different sizes. I've been looking for the slip of paper, but can't find it. I was going to do a comparison fitting them all in the same barrel and measuring the protrusion much like Peter spoke of above to try to get to the bottom of the different size/ different datum issue. Might have to gather them all up and do it.

[jmoore]

Additionally, there's gages that can be had in 0.001" incremental steps for this/these chambers!

http://www.brownells.com/gunsmith-to...0.aspx?ttver=1

All I know is that 7,62 NATO CARTRIDGES tend to run longer at the shoulder than many .308 Winchester cartridges.

Then there's the minor mystery of why the neck dimension for the chamber is about 0.005" larger than most any other .30 caliber round. It looks like someone left off a "0" so that 0.3406" became 0.346"...

[enscien]

The 0.400 datum diameter I quoted for UK military 7.62mm NATO weapons and ammunition is taken from the NATO standardisation drawings which were based on the original American design. All NATO countries may be expected to use this for military purposes but as Peter says, will have different headspace criteria for different weapons.
Dimensions for the .308" Winchester chamber and cartridges are given in SAAMI drawings published in the American NRA 'Handloading' manual and also use the same .400" diameter datum. Limits of 1.640/1.630" are given for the weapon, and 1.634"-.007" for the cartridges. Interestingly this datum is not reproduced in the Imternational CIP tables (Table I) for .308" Winchester.

[Thunderbox]

The 0.400 datum diameter I quoted for UK military 7.62mm NATO weapons and ammunition is taken from the NATO standardisation drawings which were based on the original American design. All NATO countries may be expected to use this for military purposes but as Peter says, will have different headspace criteria for different weapons.
Dimensions for the .308" Winchester chamber and cartridges are given in SAAMI drawings published in the American NRA 'Handloading' manual and also use the same .400" diameter datum. Limits of 1.640/1.630" are given for the weapon, and 1.634"-.007" for the cartridges. Interestingly this datum is not reproduced in the Imternational CIP tables (Table I) for .308" Winchester.

I thought that it was this 0.4" datum that differed between 7.62mm and .308W, hence the difference in length between 7.62mm and .308W headspace gauges that are nominally of the same length?

[Bruce_in_Oz]

There are two very useful diagrams to be found in Jerry Kuhnhausen's book, "The US .30 Calibre Service Rifles". See pages 43 and 44.

Specific data for the M-14:

Cartridge; from head to the 0.400" datum on the shoulder: 1.634"with a MINUS tolerance of 0.006".

Chamber: from face of a COMPONENT, not TEST bolt, to the 0.400" datum on the shoulder: 1.6355" Min. to 1.6405" Max.

Thus the rifle SHOULD work, and work safely, with cartridges as short as 1.628" (to datum) in chambers as long as 1.6405". That works out to a maximum "slop" of 0.0125", which is a lot by "benchrest" standards, but this ain't benchrest.

Furthermore BOTH the .308Win and 7.62NATO use the same datum diameter on the shoulder; 0.400".

However, the .308Win cartridge dimension from head to datum is 1.634Max, 1.627"Min.

The .308Win chamber to the same 0.400" datum is 1.630" to 1.640".

Thus, it is POSSIBLE for "in-spec" 7.62NATO ammo to be too long for a min-spec .308Win chamber.

Does anyone have the related "workshop" data for the Rem 700 based, 7.62NATO M-40 sniper rifle?