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Thread: Chamber Pressure/C.U.P

  1. #1

    Default Chamber Pressure/C.U.P

    Question:

    When you see the measurement, described as C.U.P., how accurate doe's this reflect actual pressures for firearm chamber limitations?

    Any insight?

  2. #2
    Member 8x57 Mauser's Avatar
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    It should be every bit as accurate as any other pressure measurement. But it doesn't compare to the more common PSI measurement. It's just a different unit.

    PSI is pounds per square inch.
    CUP is copper units of pressure.

    As I understand it, CUP measures how much a small copper ingot of precise dimensions is crushed by the pressure.

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    Member Float Pilot's Avatar
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    I found a couple mathematical formulas to convert the two, but it was a little hard to follow. I called Winchester / Browning and they said there was not good conversion method... go figure...
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    Quote Originally Posted by Float Pilot View Post
    I found a couple mathematical formulas to convert the two, but it was a little hard to follow. I called Winchester / Browning and they said there was not good conversion method... go figure...
    Yeah, I read that too. CUP, or Copper Units of Pressure, and the PSI results cannot be converted accurately. With some cartridges the CUP and PSI are the same or close, and with different cartridges, there is a big difference.

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    The only way to accurately see a conversion is to have a reloading manual that has tested the same power/bullet combo's with both methods.
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    Quote Originally Posted by oldfordjr View Post
    Question:

    When you see the measurement, described as C.U.P., how accurate doe's this reflect actual pressures for firearm chamber limitations?

    Any insight?

    There are many different forms of preasure testing and the most common being the C.U.P. Their is a ton of information on the net about testing by the cooper crusher method and that is what most shooters are most familure with. So much for my spell checker? I really don't know of a more expensive method of doing the job but by this method?
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  7. #7

    Default L.U.P

    There is also the Lead Unit Pressure, which was used for the lower pressure rounds. The British may have used a method where they actually measured the bolt thrust. I believe both CUP and LUP are considered obsolete these days except for certain situations. This is because they just take a "snap shot" of the pressure and cannot accurately reflect the whole time/pressure curve of internal ballistics. This is why most of the reloading manuals had changes in them after they went to the piezoelectric transducer method which now gives us a PSI figure. Now, they can see the fast momentary spikes in pressure that the old method could not record. Because of this, be careful using older reloading manuals.

  8. #8
    Member marshall's Avatar
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    Page 3-6 of the following link has some data that you can see. The other pages discuss the history and the differences.

    http://www.shootingsoftware.com/ftp/psicuparticle2.pdf

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