procedure for measuring case volume

Discussion in 'Reloading' started by rscott5028, Apr 1, 2012.

  1. rscott5028

    rscott5028 Well-Known Member

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    I understand the relationships and distinction between case weight and volume. So, I'm not looking to debate that.

    If you measure case volume, what's your procedure?

    Thanks!
    -- richard
     
  2. Trickymissfit

    Trickymissfit Well-Known Member

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    I keep a log book that has case volume measured in grains of water. My method is pretty simple in that I measure full length resized cases that have a primer in them. I fill them with water, and let them stand a couple minutes (maybe tapping the side of the case a couple times to make sure there's no air bubbles). Then I add enough water to reach the very top, and measure. I've also done the samething by first measuring the weight of the case, and then filling it and weighing again. Either way works well.
    gary
     

  3. rscott5028

    rscott5028 Well-Known Member

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    Do you add alcahol or detergent to the water? I've seen references to "a little". But, I'm not sure what "a little" is? Or, what specific detergent?

    For clarity, I don't care what the actual volume is. I'm interested in culling brass to form a consistent lot for competition.

    Is it worthwhile to seat spent primers upside down?

    thanks!
    -- richard
     
  4. WapitiBob

    WapitiBob Well-Known Member

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    the detergent breaks the surface tension of the water so it doesn't form a "hump" at the case mouth. A few drops in a cup of water should do it. The primer pocket will need to be the same on all, however you do it.
     
  5. Bart B

    Bart B Well-Known Member

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    For me, the best measure of case volume is case weight. I think case volume is best measured when the case walls are pressed hard against the chamber walls and bolt face; that's when the case outside dimensions are exactly the same for each case. And that's also when chamber pressure is at its peak. Otherwise, the normal varables in outside case measurements will effect the space inside of the case. While there may be very small differences in muzzle velocity in cases sized to different dimensions, I don't think they're enough to matter. And the only way to measure them accurately is with the rifle clamped in a machine rest free from a human's inconsistant ability to hold the rifle into their bodies the same way for each shot.

    Do this for a detailed result:

    1. Calculate the chamber volume in cubic inches with simple high school math formulas for cones. You might need a chamber case to get the dimensions. Otherwise, use SAAMI's chamber drawings for reference; they're close enough for factory rifles.

    2. Multiply the chamber volume in cubic inches by 1597; a cubic inch of cartridge brass (70% brass and 30% zinc) weighs 1597 grains.

    3. Weigh cases to the nearest 1/10th grain.

    4. Subtract case weight from chamber volume weight in brass; the answer is case volume in brass weight.

    5. Divide case volume weight in brass by 1597 and the answer is case volume in cubic inches.

    (I think I did the above right..........)

    Why is case volume important to you?
     
  6. rscott5028

    rscott5028 Well-Known Member

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    I have 300 pieces of once fired 6br Lapua brass. And, I want to find the most uniform 50 for a competition.

    I figured I would go ahead and check volume and see how much variance there is. If they vary a lot as a percentage of case volume, then it may be useful. If they don't vary so much, then it might be worth a quick check just to cull a couple of questionable cases.

    I also plan on sorting bullets by bearing surface length and a few other things above and beyond what I would do for a long range hunting load.

    Either way... If I've wasted my time, I'll know soon enough.

    Thanks!
    Richard
     
  7. Bart B

    Bart B Well-Known Member

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    Weigh 'em and sort 'em into batches with a 1 grain spread in weight.

    There's a greater difference in internal ballistics from exact charge weights' normal variables in peak pressure than what actual volume difference there may be. Besides, if their outside dimensions are not all exactly the same, the volume differences using the water method won't be very accurate.
     
  8. rscott5028

    rscott5028 Well-Known Member

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    BartB,

    On your recommendation then, I may well do both methods ...at least this time around.

    Worst case may be that I've wasted some time tinkering.

    Best case may be that I eliminate a few fliers?

    thanks!
    richard
     
  9. Bart B

    Bart B Well-Known Member

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    Most fliers are caused by the shooter. In second place, the cause is unbalanced bullets; sometimes new ones are that way and other times they're mangled by reloading tools and processes.
     
  10. Mikecr

    Mikecr Well-Known Member

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    I agree and disagree so far..
    For H20 capacity the cases need to be fully prepped to uniformity, and fully fireformed(unsized).
    I also put just a few drops of alcohol in a cup of water to reduce meniscus at the mouths.
    I don't clean cases to squeaky clean or anything.
    I stand a deprimed case on a plastic golf tee inserted into the flashole on a scale. Zero the scale, then eyedrop water in to fill to the mouth. If needed, I touch a tissue corner to flatten water. Record the weight, update QuickLoad file.
    It is a good idea to go slow at this.

    You can find cases that are not playing with the others, and you can also see clearly that case weight itself does not directly correlate to volume.
    Initial confinement plays a big role in powder burn and where and how peak pressure occurs. Believe, don't believe it, but be sure to see it across a good chronograph.
     
  11. Bart B

    Bart B Well-Known Member

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    How does one fully prep their cases to uniformity?

    They still won't all have the same outside dimensions even after firing them in the same chamber.

    I don't believe you'll get the same results with each case over several firings. Have you repeated your tests at least 3 times with the same batch of cases and got the same answers each time for each case?

    Proof of a test being a good one is every time its run, the same answer is obtained.
     
  12. rscott5028

    rscott5028 Well-Known Member

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    It may be increasingly difficult to spot outliers after the first culling with these small/efficient cases.

    My expectation is that after the initial culling, the brass will behave consistently with consistent treatment.

    But, I'll certainly keep it in mind and monitor.

    Thanks!
    Richard
     
  13. Trickymissfit

    Trickymissfit Well-Known Member

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    I never have, but if I were to do that I would add "Photoflow." A drop of Photoflow is more than enough. I just leave the spent primer in the case. I never gave all this a lot of thought. But I did have a buddy that actually made some nylon plugs, but that was a lot of work for little gain in my book.

    You can buy Photoflow at most camera shops, and a pint will last a very long time! (less than $10). It makes water seem like it it's wetter and also makes it spread out better on a surface. I use it to help me clean vinyl records, and it really helps.
    gary
     
  14. Trickymissfit

    Trickymissfit Well-Known Member

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    many years back I ran a test on brass to see if it had different weights for a specific cubic inch of size. I found it varied all over the place (was not being measured for reloading). If you used only one brand and lot, I think you'd be OK by case weights. But most of us don't. I ran this by a well known metalugical engineer, and he laughed at me (I always thought I was pretty good at metalurgy, but he was a lightyear ahead of me). He said everybody had their on make up of the alloy, and thus the weight per a certain volume would vary from brand to brand.
    gary