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Thread: tough qustion

  1. #1
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    tough qustion

    I'm not sure how to even ask. When a cartridge goes off, there is a pressure spike. Does,anyone know if the pressure is the same through out the case. In other words, is the explosive force the same just above the extractor groove as it is in the shoulder?

  2. #2
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    Quote Originally Posted by Dave B View Post
    I'm not sure how to even ask. When a cartridge goes off, there is a pressure spike. Does,anyone know if the pressure is the same through out the case. In other words, is the explosive force the same just above the extractor groove as it is in the shoulder?
    Wow, you're getting into the theory of infinitesimals as pertains to the internal ballistics of a rifle case. I suppose it all depends on how small a time increment you're using. If the time measure is a milli-micro-nanosecond, than the pressure is probably not the same.

  3. #3
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    clarification

    I said it's hard to explain....often we stretch a 6 mm neck to 30. Just stretching the brass thins it. Yet normal use if you pay attention you never blow out or split the neck. Same goes to changing shoulder angle or body taper. If you stretch the brass and it has plenty of chamber support there is seldom a problem.
    I think most of us will agree here. My question go to be body of the case. How far can I stretch it? How far toward the extractor groove? Why? I'm using lapua brass and as of yet there has been no issues. Luck, karma or stupidity.

  4. #4
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    Quote Originally Posted by Dave B View Post
    I said it's hard to explain....often we stretch a 6 mm neck to 30. Just stretching the brass thins it. Yet normal use if you pay attention you never blow out or split the neck. Same goes to changing shoulder angle or body taper. If you stretch the brass and it has plenty of chamber support there is seldom a problem.
    I think most of us will agree here. My question go to be body of the case. How far can I stretch it? How far toward the extractor groove? Why? I'm using lapua brass and as of yet there has been no issues. Luck, karma or stupidity.
    If I remember around .150 of unsupported case is around the max. hope this answers what you are asking..

  5. #5
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    thanks George

    I knew someone would know. Sometime ago I had Dave Kiff make me a short 22, 6mms and 30 cal reamers all on various shortened 284 cases. Two of these shot way over my expectation on didn't. The cases are tough to build because the brass is a lot thicker as you move down the case. To form them correctly I first turned them into astraightened tube, thinned the brass and then completely reformed them. As you can imagine, this is quits a job.
    One day I ask myself why I didn't just blow out a shortened 6 br case? I did and it looked fine. Certainly fat, but fine.I made 20 more and never lost a case. I shot a registered HV match with them and shot a 14 agg. Last night I shortened a 6.5x47 lapua case and fired it in a short 30x47. I looks exactly as I want it to.
    My wife occasionally says "just how stupid are you?" I trying to find out. What are your thoughts?

  6. #6
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    I can't imagine how the pressure could possibly be the same in the entire rifle/case/chamber deal at all times but it is. So, to the original question, I would have to answer yes - the pressure is the same throughout.

  7. #7
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    Quote Originally Posted by Wilbur View Post
    I can't imagine how the pressure could possibly be the same in the entire rifle/case/chamber deal at all times but it is. So, to the original question, I would have to answer yes - the pressure is the same throughout.

    Keep in mind that after ignition the volume is constantly changing. Within any given volume the pressure would be the same as near as scientific measurement could be made.

  8. #8
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    Though it is a stretch to apply it in a cartridge case, the Bernoulli equation says that pressure is higher at the base of the case where the velocity is zero, compared to in the neck where the velocity is higher. Also, to accelerate the combustion gases within the case, the pressure must be higher at the base. These effects are small in comparison to the total pressure, though, so I agree with Wilbur!

    Keith

  9. #9
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    hard question #2

    When I first started this game the sportier class has just barely started. We shot 6x47's made on 222 mag cases. It worked for a 22 why not a 6 ? They shot really one day and 24 hours later they didn't shoot at all. When the ppc's came out I had one within the first 9 months. It solve most of our problems but maybe not all. The tune I have here in the desert may not shoot as well in Ohio. As aggs have shrunk over the years you see the need to tweek your load all day to keep your tune. While we're certainly better off than 6x47 days I don't feel we've completely solved the problem. My 30BR seems to shoot the same tune wherever I go. So does a short 22PPC, so why not a 6PCC? I'm not trying to find a case that shoots better than a PPC,
    I'm just trying to find one that shoots as good all the time.

  10. #10
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    I think there are 6PPC rifles that shoot as you wish. Getting one of those is the entire game if you run it to ground.

  11. #11
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    Wilbur

    Why not try to build a rifle and cartridge that stays in tune the majority of the time all by itself. Like a 30 BR seems to do at 100?

  12. #12
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    Just the detonation of the primer alone is enough to cause pressure waves to propagate throughout the case/bore. I think those cases that shoot well are doing so,in part because of the way they manipulate these waves.

  13. #13
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    Expansion ratio...small case, big bore.

  14. #14
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    FWIW the exhaust system on a 2 stroke engine (called an expansion chamber) behaves much the same as internal ballistics when it comes to pressure waves, they even share the same shape. The cone at the back of the pipe (converging) reflects pressure waves back into the combustion chamber stuffing fresh charge back into the chamber thus giving a supercharging effect although to a much lesser degree than a proper supercharger. The cone degree and pipe internal length are designed to provide this boost at a particular timing to fill in low torque output.

    All done to manipulate pressure wave timing. The length of the case in relation to the length of the barrel can thus act like a muzzle tuner.

  15. #15
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    Quote Originally Posted by Dave B View Post
    Why not try to build a rifle and cartridge that stays in tune the majority of the time all by itself. Like a 30 BR seems to do at 100?
    I think that's what I've been trying to say...for years and years!

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