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Thread: Very Confusing GP-11 and Go Gauge and Case Headspace Differences 3 Rifles (phew)

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    Really Senior Member RC20's Avatar
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    Very Confusing GP-11 and Go Gauge and Case Headspace Differences 3 Rifles (phew)

    I am looking for an explanation as to why GP-11 cases on the shoulder ala where head space is determined are so SHORT?


    I name my rifles so the K-31, the 1911 are a given. My 7.5 is a Lother Walther barrel so its the 7.5 LW though its a 7.5 Swissicon just like the K-31 and 1911. Its built on a Savage receiver and you set your head space which I had done and thought well done.

    I own a LabRadar and I wanted to see how a GP-11 FPS was in the K-31 and the 7.5 LW (I had what should have been a safe reload in the 7.5 LW go seriously over-pressure)

    The 7.5 LW would not fire the GP-11, dent the primer yes, fire no. Non fired round would not extract, ergo, head space way to loose (which was a ??? as no issues prior). The head space was clearly too much and should not have been.

    So I took it home and began to measure things.

    Chambers are real close with the K-31 and the 1911 a few thousandths longer than the 7.5 LW the way its set. Should be fine.

    So I pulled a GP-11 bullet out of a case and measured the shoulder. Its .016 SHORTER than the head space gauge. Ergo, the chamber is going to be a bit more than the GO gauge.



    The GP-22 cases are short by around .016 from the head-space gauges. As the head-space is not intended to be tight (a few thousandths) that means its probably .020- .025 short.

    My take is the K-31 fires the GP-11 because the extractor holds it to the bolt and the firing pin get a solid jab at it.

    And an oddity, the latter year GP-11 of which I had a few (79) have the grip grove on the projectile further out (the two lots of the other are 77 and 78 and as you can see from the picture, that grip grove is further Attachment 114276 from the tip.
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    Advisory Panel Patrick Chadwick's Avatar
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    "Non fired round would not extract, ergo, head space way to loose"

    Sorry, I don't get that. If the round didn't fire then the case didn't expand. So why no extraction? Chamber too tight?
    Or is the extractor itself at fault?

    "I had what should have been a safe reload in the 7.5 LW go seriously over-pressure)"

    That rather suggests that the LW chamber is too tight.

    ??? I'm puzzled, could you please check your post?
    Last edited by Patrick Chadwick; 01-17-2021 at 11:08 AM.

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    Really Senior Member Bruce_in_Oz's Avatar
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    Swissicon chambers are interesting.

    The chamber on a G or K-11 has a LONG throat. This is primarily so that remaining supplies of the old "torpedo" GP-95 could be safely chambered and fired. There was also the equal "bonus; the generous freebore kept peak pressure down with the somewhat more lively GP-11.

    By the time they got to producing the K-31, the old ammo was merely a curiosity, thus these rifles have a short throat.

    As anyone who has tried to brew handloads for the K-31, using Sierra Matchkings will have discovered, the throat IS very short. SMKs have TANGENT ogives.

    The GP-11, having a SECANT ogive and a VERY short "parallel" bearing surface works perfectly with this short throat; i.e., the cunning Swiss chaps designed their new rifle AROUND the exquisite GP-11 cartridge.

    There is usually VERY little "variation" in the headspace of correctly maintained K-11s or K-31s.

    If you think you've seen some odd chambers, take a close look at the StG-57 / PE-57. Unsurprisingly, in the 1950's, the Swiss came up with an idea that has recently resurfaced in the odd little FN 5.7 x 28; "creative case bulging", to reduce back-thrust on the bolt and retard extraction a few microseconds until peak pressure has dropped.

    In pretty much all military systems, the fired brass cases are NOT reloaded, but recovered for their scrap value. This neatly avoids the potential hazards of cases "failing" on a two-way range, at extremely inconvenient moments. Steel cases, which must NEVER be used in a Sig PE57 or SiG AMT, are generally regarded as "biodegradable".

  6. Thank You to Bruce_in_Oz For This Useful Post:


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    Contributing Member Ovidio's Avatar
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    I can confirm the bulging on the StGew 57. The brass is really in a bad shape when it gets out of that rifle.
    After swapping it for a Mosin M39, I kept the brass, just for reference. If you need any pictures for any reason, just tell me.
    I'd be glad to post them.
    34a cp., btg. Susa, 3° rgt. Alpini

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    Really Senior Member RC20's Avatar
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    Sorry, I don't get that. If the round didn't fire then the case didn't expand. So why no extraction? Chamber too tight?
    Or is the extractor itself at fault?

    "I had what should have been a safe reload in the 7.5 LW go seriously over-pressure)"

    That rather suggests that the LW chamber is too tight.

    ??? I'm puzzled, could you please check your post?
    I am trying to sort through confilicting aspects, one being H4831 loads that should have been very safe were over pressure. Tight chamber (that can be diameter not length)

    That followed with, ok, lets check the aspect we can, which is really head space (and I can mike brass diamter now that I thinkk about it)

    So, item 1:

    GP-11 Case (not the bullet) on the shoulder check is very very short.

    The Head Space GO gauge is in fact what I would expect when checked in a K-31 or 1911 Chamber (never was sure how to designate the 1911, K-11 works as well)

    Failure to Fire: That is all related to a chamber or round length and at the shoulder (that is what stops it from moving forward)

    If you are way to long, when the firing pin hits the primer, the case moves forward, you likely (in all cases for me so far) have a dent. But a dent is not a fire, you need the cartridge to be held, not allowed to move. No issue it may move a bit but it can't fire if there is no shoulder contact (head space again).

    Now, there are two types of feed on guns. Controlled Round and Push feed. Military used controlled round (the base moves up into the extractor claw as it feeds). That was popular with gibve game as supposly more reliable .

    Push round just pushes it forward and then the extracor snaps over the rim as it comes to a stop.

    The 7.5 LW build is a push round action (typeil of modern actions). If it does not snap over and extract a cartridge, that tells you the head space is too long (as does the failure to fire and a dimple, you have to tap the round out with a rod). I ran into that with my first Savage build, I was being safe (or so I thought) and did not get the head space tight enough.

    But, head space also depends on the Shoulder of the Cartridge. Only a reloader would know you can change that to suit a given chamber. You fire a cartdige, measure where the index is on the sholder and then when you resize for THAT gun, you adjust the sizing die to move it back .003 (or .005). It saves on the brass and if you had a long chamber (or set the headspace long and still barley fired) the next time its spot on.

    So the variable can be the cartridge shoulder as well as the barrel setting.

    What I found was, yes the 7.5 LW has a chamber that is barely past the GO gauge (NO GO is too much). In this case its closer to the GO than NO GO which is just about perfecft (or you can try to match the K-31/1911 whihc is a bit longer, assume for combat pursposes).

    So, where is the excess lenght coming from? GP-11 fires in the K-31 fine.

    Back to that claw. In the K-31 its held in place by the claw, does not have to be up against or close to the shoulder.

    In the 7.5 LW build, even though its a tad shorter on th chamber, its not held in place and won't fire.

    Ok, what is the GP-11 shoulder measure?

    Very very short. Direct measure the GP-11 case measured on its shoulder is .016 SHORTER. Add in .002 or so normal head space past the GO gauge (you don't want to set it tight) that is pushing .020 short.

    I find that astonishing. They are counting 100% on the claw hold to fire the round.

    I had this occurr on a pistol. I had a 45 ACP semi auto, grabbed some pracit rudn for it and down at the ragne it fires, but does no cycle. Huh, this thing is spot on reliable. Another one and same thing.

    So I look at the case, what I have is 45 GAP (curse Gluck and their ego and my ignorance I did not even know Gluck had created that stupid round)

    Now its about 5 mm shorter, 45 ACP is supposed to head space on the case mouth.

    In this case, the extractor held the round well enough to fire but the cycling was mucked up. But yes it fired (and no issues with the gun fortunately). So, while technically a 9mm or a 45 ACP etc is head spaced on the mouth, the extractor will (or can) hold it and fire it if the case is short.

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    Really Senior Member RC20's Avatar
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    Well partially solved. The too hot loads were wrong powder Identification. Got that sorted and purged from the dispenser it was in.

    I did fine a 7.5 PPU New case, it matches on the shoulder the GP-11 closely, I think the contour is a tad different. Still no explanation as to why cases are so short and the chambers are relatively so long creating a head space that amounts to Field reject +.

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    Really Senior Member Wineman's Avatar
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    Old news I'm sure but when the K31icon was designed, the chamber was made wider at the shoulder than the previous GP-11 rated rifles and carbines. A case fired in a K11 still looks tapered, a case fired in a K31 looks almost straight. Most reloading dies usually take brass to GP-11 dimensions (narrower). Sometimes to get the base back to specification, the case has the shoulder set back farther than it needs (too much cam-over?) and you may get light primer strikes in a K31. If you have dies for K31 spec cases, and a K11 style chamber, the brass may not fit and extraction could be an issue. Remember a brass case is a poor gauge due to spring back, softness etc.

    Dave

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    Really Senior Member RC20's Avatar
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    Dave:

    Good info, I have a 1911 and can test it. With the 7.5 LW I have one of each of the groups (K-31, K11 and the 7.5 LW (its Loather Walther barrel what chamber do they do that in?)

    I have the chamber gauge but assume its European spec (CIP?) whatever that really is.

    I do run separate groups of brass in each one.

    I just do a minimum shoulder bump back for re-size, shooing for .004 (ideally .002 but with the case variation I get some too long so I go for .004)

    Still why the major difference in the shoulder being so short vs the chamber? As that is GP-11 and new brass that is a standard.

    The 7.5 can be set headspace wise to whatever I want as its on a Savage receiver and the barrel can be screwed in a more. But that would mean a guess or using a new piece of brass as that is less than the headspace gauge by a fair amount.

    Werid stuff.

    Have to see if I have some new 30-06 brass to look at old cartridge and see how it comes out. Some reports are that the US looked at GP-11 bullets latter as well as Mauser 8mm basic design when they did the 06.

    What is odd is that they went with a more slopped shoulder. The GP-11 is clearly a more efficient cartridge. Its clocking 2550 or so with around 50 grains of powder (R17).

    I have taken apart 4 bullets now and clearly they are adjusting powder to strength of powder batches to maintains a velocity as it ranges from 49.4 to 50.5

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