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Thread: Chamber Pressures

  1. #11
    Gunco Maniac sjohnson's Avatar
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    You're right. I was too busy trying to make the math readable and didn't pay attention to the details. My post is now amended. Thanks!

  2. #12
    Gunco Maniac sjohnson's Avatar
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    (silly double post)

  3. #13
    Gunco Regular Gunter's Avatar
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    Things to rememeber is that we don't know the type of steel and RC hardness of the bolt lugs and trunnions. We were guessing on the numbers used in Dan's program to come up with bolt thrust and flexing in the other thread. Plus, ammo makers are real tight lipped about the pressure (cup or psi) that their ammo is running.

    Like HC said, I check every case after firing with a case gauge. Then after the testing session, I check the rifle with a set of go no-go gauges. So far nothing has changed with the 7.62 Nato and the Yugo bolt/trunnion. I'm NOT saying that it is safe to build anything!

  4. #14
    No Hope For Me 1biggun's Avatar
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    SO what happens when you use a rebated case?? I suspect you still use the dia of chamber for the calculation.

    Can some one tell me the differance in bolt thrust of a 308 at 50,OOO PSI and A 7.62 X39 at 50,000 PSI and a 223 at 50,000PSI all shooting a 100 GR bullet??
    I want to know what the increase there is by going to a larger diamiter casing/rim. remember that the 223 is around 60,000 PSI for hot hand loads. I have loaded my 223 varmit AK right up to the point of blowing primmers. Im gusseing it is in the 60,000 PSI area. what is the bolt thrust that it is generating.

  5. #15
    No Hope For Me 1biggun's Avatar
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    some were there is a site saying not to excede 50,000 PSI for the 7.62x39 round. I dont trust a lot of what I read unless it is from a manufacture or a professional ammo supplier. I will try to find the sight.
    50,000 PSI would be an OK pressure for most 308 based rounds to work at. this would offer a huge improvement over the 7.62x39 round.

    TO compare apples to apples we need the bolt thrust of each round. to make this work.I cant do the math so this is were I need help. or mabe a turtorial on what buttions to bush on a calculator to figure it out.

  6. #16
    No Hope For Me 1biggun's Avatar
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    SOrry for all the posts but
    Wouldnt you want to use the INSIDE DIAMITER OF THE shell at the back to calculat bolt thrust?? I suspect a a 308 with a rim, a 308 non rimed and a 308 with a rebated case would all have the same bolt thrust. wouldnt you want to use the area inside the the case at the back (the bore of the case))to make this calculation???

  7. #17
    Gunco Regular Gunter's Avatar
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    If I remember right, Dan's program uses the inside dia. of the case head.

    So using Dan's program which doesn't have any input for bullet weight. All using the same bolt dia., lug length, width, and shear strength data for each.

    .223 at 50,000= 3539psi Bolt thrust.
    .308 at 50,000= 5825psi Bolt thrust.
    7.62 at 50,000= 4981psi Bolt thrust.

    Edit to add: .223 at 60,000 = 4246 Bolt thrust.

    The flex in the lugs were almost the same between the 7.62x39 and the .308.

    Which lug flex is one of the keys to being safe. There are differences in lug length between different AKs which effect the strength of the bolt/trunnion.
    Last edited by Gunter; 10-25-2007 at 04:06 PM.

  8. #18
    Gunco Regular Gunter's Avatar
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    Running the numbers that Vz58 used.

    7.62x39 @ 44700 cup and .308 @ 52100 cup the difference in bolt thrust was 36% higher for the .308. So, it seems that vz58 figures matches Dan's program or the other way around. But again, we don't know for sure what pressures factory ammo is running at, bolt/trunnion metal type, or shear strength.

  9. #19
    tired of idiots vz58's Avatar
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    Yes you would want the inside of the case for the thrust but I could only remember the exterior. I am getting older and more senile everyday..

  10. #20
    No Hope For Me 1biggun's Avatar
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    THere is a considerable differance between the 7.62xs9 and the 308 at the same pressure. the 308 is just over 17% higher than the 7.62x39. or 844psi higher. not a lot but a considerable amount to factor in. Very interesting. THIS IS WERE WE SHOULD HAVE GOT TO A YEAR AGO ON THE 308AK THREAD!

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