Case Weigth Vs Powder Capacity

R

RC20

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I thought this needed a slightly different look, so I took an RP case that weighed 173 grs vs an FC case that weight 181 grs and used 748 powder to fill then up level.

The powder difference was .5 grains. That was using the same mfg primer

That did surprise me, be interesting to see where you get 6.5 grs weight difference in metals that are so close.

As an aside, while 30-06 RP is just below FC on weight, in 308 its the lightest of what I have (RP, FC, PPU, LAPUA)

So far, FC has been the heaviest of various ones I have weighted (not complete by any means, 270, 30-06, 300 WM, 7mm as I recall)

Also the softest.
 
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Assuming the same dimensions on the outside, (and the same alloy of brass) the heavier case has to be thicker somewhere, so there is less capacity inside.

It varies from maker to maker, caliber to caliber and lot to lot of production in each caliber.

This is ONE of the several reasons why we work up loads, carefully!
 
Yep, this is why we sort by headstamp and work up our loads.

Here's some screenshots of some testing I did on .308 cases. Sample per headstamp was 100 cases.

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Internal capacity should be checked with a fired case in grains of H20, not powder.

Here is an article that also lists various brands of 223/5.56 brass.

http://www.6mmbr.com/223rem.HTML

Scroll down to .223 Rem Case Weight vs. Capacity

You will see that it is not always as simple as "Heavy case = less capacity" and "Lighter case = More Capacity"

The only way to be sure of internal capacity is to measure internal capacity, case weight is not completely reliable.
 
Ran into the same issue over two years ago. Reloading for 308 cal. using IMR 4064. Brass Remington & Winchester, both volumes and weight are the same. Reloads were good, then I was given FC brass from a shooting buddy that doesn't reload, brass once fired in his rifle was great. After sizing I noticed the powder was much higher in the case. Even when trimming I could see the brass around the fresh cut necks looked & are thicker. The thicker case works better for me due to I benchrest only 200 yard max. With medium loads. 4064 is a stick powder, with a medium load it has very little air space. I F/L size with the expander ball so the neck tension is the same. I like the thicker brass.
 
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This was not a super tech test of course.

But I did use 748 which is a fine grain ball powder so a few grains would not push results a long ways (4831 comes to mind, 3 grains and you have grain of weight as it were.

I did find it interesting that despite the weight different the powder difference was small (larger case rifle anyway)

Might be enough to push you over the edge but not a whole lot.

Surface tension on water is dicey as well, and powder is what really counts as I have yet to shoot a round with water in it (grin)
 
The traditional method for measuring case volume is with water, to the base of the case neck. This provides for a very uniform and consistent standard.

A fine powder like 748 will vary a lot less than an IMR stick powder but the ball powder will still have more weight variance per volume than a liquid such as water.

plenty good enough for a rough estimate, though.
 
Reloading is technical in every way. Even using a Redding standard shellholder & a standard RCBS she'll holder there is a difference in die setup height.
 
cw308 said:
Reloading is technical in every way. Even using a Redding standard shellholder & a standard RCBS she'll holder there is a difference in die setup height.
Click to expand...



Absolutely. I ran into this early in my reloading days. I have RCBS and Hornady shell holders. I have to make good notes to which one I use at which stage of reloading.
 
The traditional method for measuring case volume is with water, to the base of the case neck. This provides for a very uniform and consistent standard.

A fine powder like 748 will vary a lot less than an IMR stick powder but the ball powder will still have more weight variance per volume than a liquid such as water.

plenty good enough for a rough estimate, though.
Click to expand...

Not trying to be argumentative, but having done a bazillion battery readings,
I would argue trying to figure out where the water line should be is also an issue. I think the fine grain powder test is probably as good, neither is in depth accurate.

Filling it with water and then using an eye dropper to get to overflow might be the best between the two (though how to move it to a scale?)

Or really fine grain substance.

Ok, I ramble, this ying and yang of technical vs people who just do a volume load and shoot very tiny groups indeed. I get mine down to a grain, but only because it makes me feel better, not because it makes a difference.

Setting back shell with a shoulder measurement now as well, RCBS has a hard cam over, but that break cases someplace round 8 or 10 reloads.

What the fine grain test does is establish something in the 1/2 grain powder reality, and that is what we are about, how does it reflect with powder, not water

As 3/10 one way or the other does not (per the experts) then it gives the relevance we need. Probably case soring does not good per that.

Case sorting may get better results as the case expansion characteristics and how it functions in the chamber might, but powder variation it seems to be no.

For what I can shoot I suspect it irrelevant, still makes me feel better so I do it.
 
With water, fine grain or stick powder, once you notice a difference its up to you to make the adjustments. How you do it, well I guess everyone has there system.
 
I don't have the Quickload type software.A brother does.What he had me do to calculate the case volume to plug into the software,is file a flat on a bullet so it could leak water.
Then weigh the bullet and case(with a spent primer installed) dry.
Fill the case with water,then seat the bullet to LOA (slowly)
Then weigh it.True combustion chamber volume,no meniscus to mess with.
In theory...remember Archimedes ,charged with determining if the Goldsith who made the crown corrupted the alloy to rip off the king.Archimedes was taking a bath,it occurred to him he could use water displacement per weight to determine the alloy purity.
So,in THEORY,if the outside geometry were identical case to case,and if the alloy were identical case to case,
We could use weight to compare and calculate case volume.
But the problem,there is subtle geometric difference brand to brand.Rim and extractor groove for example.
And not all cartridge brass metal is the same.Variation in alloy will result in a different specific gravity.

Any measuring process introduces some error.Thats why we have a tape measure six in scale ,a dial cal,a micrometer,and maybe a Swede comparator stand,an indicator that will measure 50 millionths or better,and Johannsen blocks.
 
A rule of thumb come up with by Wm. C. Davis, Jr. long ago was that it took 16 grains of case weight difference to require a 1 grain difference in powder charge. I've found 14 grains of brass with some powders, but it's an interesting fact that these relationships hold up across cartridge types and peak pressures pretty well.

Davis, too, was assuming identical exterior dimensions with zero tolerances, which seldom happens. But the main takeaway is that if you can't measure powder more exactly than 0.1 grains, any brass weight difference smaller than 1.6 grains can't disturb things any more than 0.1 grains of powder charge error does.
 
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