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

[jmoore]

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

I might be able to find out. I do have a Quantico 2112 built rifle, but don't have the full set of 0.001" incremental gages. Whether my informed contacts have the info locked in their brainpans, I don't know. But, it seems that they weren't anything special, IIRC. There also have been some rifles built by subcontractors in small numbers during the M40a3 development phases.

The case neck diameter issue mentioned above is of more than academic interest, as well. There are chambering reamers that are in line with the old 30-06 diameter specs which are used not infrequently by some competition shooters on the high power circuits. Loose enough to be safe and reliable with semi-autos, but providing better case alignment and far less case neck expansion.

[jmoore]

Just heard back from a friend. No special setting of headspace on the M40s of which he's aware.

[Peter Laidler]

I always had an inbuilt suspicion that we/Armourers and the worlds shooting fraternity always pout far too much emphasis on CHS. I had an old No4 trials rifle that you'd never get to CHS in a zillion years, no matter how many bolts or bolt heads you tried. The locking shoulders in the body were absolutely worn out, It was over .010" excess - and probably more - but it still shot and grouped well. I only abandoned it when I started getting light strikes which I put down to the excess CHS.

Some years later I read Hatchers writings of CHS and he seemed to agree. Naturally, when it's going to interfere with safety or the structural integrity of the weapon, then it's something else of course. But I recall otherwise good GPMG's being scrapped for worn locking shoulders. After reading the Hatcher paper I submitted a comprehensive report that such wear, in an expensive weapon might be acceptable for a further .080" to .010" after a structural NDT examination and code-marking of the body of the body to this effect. Didn't hear any more!

Worse still, was scrapping No8's for excess CHS too............ I mean............... scrap for excess CHS on a .22" rifle................!

[jmoore]

The real drama with excess headspace on a military weapon is the serious stoppage that occurs with casehead separations. The more serious problem it seems to me is insufficient headspace.

[enscien]

What excess CHS is acceptable depends on a number of factors. particularly the amount of support given by the weapon chamber and the strength and lubrication of the cartridge cases used. As ammunition designers at Enfield we used to test 7.62mm cartridge cases to destruction in an L4 Bren barrel at CHS values of 1.630, 1.640, and 1.650". Rounds were handloaded to produce 3 different pressures from service to proof levels, oiled, and fired at increasing headspace settings (from a proof housing) until there was significant expansion of the cartridge case rim and sometimes loose caps. The L4 Bren barrel with its chamfer and extractor slot gives less support than the L7A2 GPMG, so was a better test vehicle.
For a target or sniper rifle tight headspace is desirable so that the round is centred up in the chamber. The Enfield 'Envoy' target rifle had a specified CHS of 1.628/1.631" whilst the British 7.62mm L2A2 Ball round has a specified value of 1.628/1.634". So a high round would have 0.006" crush-up in a low chamber. (I believe that originally they even went down to 1.625" in the rifle, but that started to make the bolt hard to close.)

[Simon P]

CIP Link to the .308 Win

http://www.cip-bobp.org/homologation...en-page125.pdf

[Parashooter]

CIP diagrams are interesting in expressing the headspace dimension as the shoulder cone angle and the distance of the cone's apex from the cartridge base or bolt face.



Solve the triangle for "x" to a given datum, subtract the result from the base-apex dimension ("S" on CIP drawings), and you have an equivalent to the SAAMI or NATO base-to-datum dimension.

Note on the referenced drawing that "S" is .1mm (.004") greater for "CARTRIDGE MAXI" than for "CHAMBER MINI", giving almost the same "crush up" as described above for the Envoy.

[Bruce_in_Oz]

Enscien, you just reminded me of the great "L-42 in Oz" caper.

I had nothing to do with the testing and evaluation of the L-42 in Australia, but later hear some strange tales.

The "BIG" one was the issue of headspace.

I don't know what gauges were being used, but it seems all and sundry were perplexed by the "odd" headspace and thus the "concensus" was that there was "something very wrong" with the rifle.

I sometimes wonder at the bizarre internal conflicts, (and results), when diverse philosophies, "new-toy lust", compartmentalised learning and limited "externnal" experience come into play simultaneously on a military procurement, or any other, project.

A lot of Benchrest folk set things up so that there is virtually zero headSPACE so that the shoulder cone is fully seated on the chamber cone. Note that unlike the Lee Enfield, P-14 / M-17 and Garand/M-14 actions, most "precision" rifles do NOT have helical locking, thus they do not have the ability to slightly compress the brass case into such a position. Note that Swiss Schmidt Rubin derived designs have helical locking/unlocking out of necessity. They seem to shoot pretty well too.

Interesting to note also that in designs with plunger ejectors, especially self-loading types, the cartridge shoulder encounters the chamber shoulder well before the extractor, which has also been part of the "ramming", clicks over and engages. Anyone with the right gear out there ever tried to measure the compression, if any, of the case in these actions?

Finally, in actions with spring-loaded ejectors, the cartridge is forced forward, AWAY from the bolt face, thus potentially exacerbating headspace/striker protrusion issues. When a cartridge with a non-crimped primer is fired, the first thing that happens is that the primer effectively blows the case forward, leaving the primer slightly proud of the case head. Once the propellant ignites, it pushes the case back against the bolt face, compressing it, whilst simultaneously blowing the rest of the case outwards/forward. The pressure keeps the brass stuck to the wall of the chamber until the bullet leaves the muzzle, thus releasing the pressure. The whole ultimate point of the case is to provide that seal, "obturation", without which shooting a breech-loader would be MUCH more exciting.