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Thread: Chamber Pressures

  1. #1
    Happy Camper hcpookie's Avatar
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    Default Chamber Pressures

    I wanted to post the beginning of a "quick-view" for some of the pressures were are working with on the military rifles.

    NOTE: This is NOT AN AUTHORITATIVE REFERECE! REFER TO THE FACTORY-PUBLISHED DATA FOR ABSOLUTE VALUES!!! YOUR SAFETY COULD BE AT STAKE IF YOU DO NOT HEED THIS DISCLAIMER!

    OK, now that that is over




    First, a brief description of CUP pressure and what that means. The short, abridged version is that a test chamber is prepared with a calibrated bore that is drilled into the chamber. A calibrated piece of copper is placed in the hole to plug it up.

    The "proof" test rounds (super-hot greater-than-magnum loads) are fired and the copper plug is removed. Proof rounds are usually beyond the normal range of reloading data since they use "excessively hot" powders for a particular caliber.

    The amount of expansion/stretching that the copper plug endured is measured and assigned the "CUP" pressure. "CUP" = Copper Units of Pressure. This is usually considered the maximum safe loads for a chamber, and exceeding these loads can cause metal fatigue or possibly failure of the locking mechanism.

    Some references:

    http://www.chuckhawks.com/pressure_measurement.htm

    http://stevespages.com


    I found the best places (so far) for published pressures are reloading reference sites. Here's what I found so far... note these are not for a particular load or powder, but the absolute largest and smallest values that are published. I would recommend more research for a particular bullet weight that is considered for a custom build.

    http://data.hodgdon.com/main_menu.asp

    7.62x39: 30000/40900 cup
    7.62x54R: 38000/46400 cup
    .308: 37000/58800 cup
    .30-06: 39000/58800 cup
    8mm: 36000/49400 cup
    9.3x62: 38500/48200 cup
    9.3x74R: 31900/42400 cup

    So far, it looks like the .30-06 / .308 is the absolute hottest caliber that was built on an AK platform. 8mm doesn't seem so scary after looking at these numbers!

    I'd like to see some more data to compare, because I hate using just one source. Feel free to contribute! Let's make this a larger list.
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    Gunco Regular Gunter's Avatar
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    From the 48th Lyman:

    7.62x39 27400/44700cup w/123 gr
    30-06 30000/57000cup w/125 gr
    308 29500/52100cup w/125 gr
    8mm was mixed with both Cup and PSI

    Trying to keep bullet weights around the same.

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    Gunco Member jimmy_stikx's Avatar
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    So doing an 8mm conversion on a FAL would be easier on the reciever, interesting.

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    No Hope For Me 1biggun's Avatar
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    On the 8mm keep in mind that the US comercial is loaded very light. the european stuff is every almost if not as hot as a 30-06. this is due to so many pre 98 mauser actions out there. the europeans load for the modern guns.

    On the 308 versus the 7.62 nato/51 ther is a differance in the SAAMI specs of the two rounds and it was some thing like 10,000 PSI differance the nato being loaded lighter.

    On the 223 vs the 5.56 the 5.56 it hotter By A bunch

    HC good Thread !!I was writing some numbers down for something simular when I posted on the 260 thread about the 308 loads.
    Bolt thrust is the measurement that we really need to know for each round as rim DIA affect it for example a 223 at 50,000 psi is not the same as a 308 at 50,000 psi. I think the formula for it is posted in my 308 AK thread some were as is the nato vs 308 info..
    It would be really nice if some one had all the SAAMI specs to post.!!!!! many of the books I have excead the those specs as hand loads are often used safely in heavy duty bolt guns. the 48TH edition LYMAN is a good example, shows 8mm loadsin the 48,000 psi range but the SAMMI spec for that round is 35,000 PSI so all USA AMMO is at or below that spec

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    tired of idiots vz58's Avatar
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    taking into account the rim diameter of the rounds and the cup involved the force on the bolt lugs would be:
    308 would be 9580cup/sqinch
    762x39 would be 7011cup/sqinch
    this would be a 36% increase in force against the bolt lugs.

    this is why I worry about longevity.
    Last edited by vz58; 10-25-2007 at 07:08 AM.

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    Happy Camper hcpookie's Avatar
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    VZ you mean 36% increase, right? How are you computing those numbers? I am calculating a different number. Thanks to Gunter, I'm multiplying the:

    Lyman CUP numbers * rim diameter and get:

    7.62x39: 19980.9
    .308: 24487

    Now dividing the numbers by each other, I am getting:

    .308 = 1.225x (or 22%) > 7.62x39

    Reversing those numbers, 7.62x39 reaches only 81% of the .308 pressure. (aka 19% < .308).

    Remember all of these are the maximum values, not necessarily what my ammo reaches. So 22.5% "max" pressure increase is what I'm putting on the lugs by my measurements. That isn't too bad... ??? I expect to use only reloads in it and not the "hot" surplus ammo (which is going into my MG42 pile!) so I should not expect to reach the max with the reloads. Lyman says 48800 with the powder/bullet combo I plan to use. That puts me just over max CUP for 7.62x39 and maybe that's as far as I need to look.


    From the discussion WAY BACK IN THE DAY in that long .308 AK thread, I remember:

    - the photo comparisons of the Saiga .308 and other bolt heads showed surprisingly similar design
    - the bolt thrust discussion, but I don't remember the numbers
    - the amount of stress tool steel could take (I forget those pressures), but it was somewhere way up there beyond anything we were dealing with.
    - CUP pressure discussions

    It was at that time that I reached the conclusion a .308 build on AK parts was doable. I would have to re-read the .308 thread to get exact numbers. Maybe biggun could run that little widget to throw out some bolt thrust numbers???



    Now, considering the chamber is mere inches from my nose on that .308 bullpup, I am taking a page from the reloaders' troubleshooting notes I've read. From a certain point of view, I am shooting "hot" ammo if you look at it as a 7.62x39 platform.

    Therefore I am on the lookout for any telltale signs of "hot" loads, such as a sticking bolt, case bulges, signs of peening, excessive recoil, etc. etc. Anything that says "its too hot" will stop me using that particular ammo. And I am checking headspace each and every time I go to the range. So far, nothing shows it is being stressed. But I'm still checking!


    I think we should compute some bolt thrust numbers... or have we already done that elsewhere? 1Biggun?
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    Happy Camper hcpookie's Avatar
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    Adding Bert01's and Gunter's calculations about bolt thrust:

    308 ak

    http://www.gunco.net/forums/showthre...ust#post373199
    Gunco Member #10

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  8. #8
    tired of idiots vz58's Avatar
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    I took the diameter of the case base, found the area that the pressure is working on and multiplied by the pressure. I used the hot loads from each. So yes 36%

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    Gunco Maniac sjohnson's Avatar
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    The pressure per square (inch, millimeter, foot, whatever) is what's important.

    The diameter is of a case rim is DIRECTLY related to the area of that rim (square inch, millimeter, etc), so you're OK in using that,

    -but-

    the real values should be expressed in CUP or PSI per square inch or millimeter, not some vague, relative dimension.

    Use (half the diameter) times (half the diameter), times PI to get square inches or square mm. PI is 3.14.

    Then, when you multiply CUP or PSI by THAT value you get pounds or kilograms of bolt thrust. A value with some meat on it.
    Last edited by sjohnson; 10-25-2007 at 08:15 AM.

  10. #10
    tired of idiots vz58's Avatar
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    sjohnson, the force should be the pressure TIMES the area it acts upon, not divided.

    from memory (correct me if I am wrong)
    rim dia of 762x39 (.447)/2=.2235
    .2235x.2235=.0499523
    .0499523x3.14(pi)=.1568502 (square inches of case head)
    .1568502x44700(cup)=7011.2039 force upon bolt

    308 rim dia .484 (or is it .473?)
    .484/2=.242
    .242x.242=.058564
    .058564x3.14=.183891(square inches surface area)
    .183891x52100=9580.7211

    9580/7011=1.36 or 36%increase

    I realize PSI would be more appropriate but all that has been provided is cup

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