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procedure for measuring case volume

 
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  #1  
Old 04-01-2012, 10:54 AM
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procedure for measuring case volume

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
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Old 04-01-2012, 12:05 PM
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Re: procedure for measuring case volume

Quote:
Originally Posted by rscott5028 View Post
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
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
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Old 04-01-2012, 01:12 PM
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Re: procedure for measuring case volume

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
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Old 04-01-2012, 03:11 PM
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Re: procedure for measuring case volume

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.
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  #5  
Old 04-01-2012, 06:18 PM
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Re: procedure for measuring case volume

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?
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Old 04-01-2012, 07:29 PM
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Re: procedure for measuring case volume

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Originally Posted by Bart B View Post
Why is case volume important to you?
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
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Old 04-01-2012, 07:36 PM
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Re: procedure for measuring case volume

Quote:
Originally Posted by rscott5028 View Post
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.
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.
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