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Old June 3, 2012, 08:24 AM   #1
mehavey
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Case Weight vs Volume...

There has been some considerable discussion on another board as to case weights between
brands and lots, as well as the inevitable so-what-to-do-about-it mantra.

On a whim I looked at three types of 308Win I have downstairs and decided to run a correlation
between weight vs volume. Since I tend to "standardize" rather that do a lot of range pickup,
my choices are limited -- but do run a fairly broad variation.

I already knew there was no on-to-one relationship in weight/volume, but was a bit surprised to see the linearity.


Anyone else have data (case-with-spent-primer/water weight) I can add to this?






post: The import to most reloaders is straightforward. If I'm running a 175SMK over 42.4gr IMR4895 in a Winchester case, I get my desired 2,550fps match velocity for a leisurely 50,500psi (on a 60° day)

If I put that same load in my LC`81 cases having 6½% less case volume, I'm not only suddenly 100fps faster, but at 60,000psi

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Old June 3, 2012, 08:55 AM   #2
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more to it too

There is also the alloy of the case, which has its own effects.
There is also the shape of the volume; ditto.

Art vs science?


I know not.
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Old June 3, 2012, 09:11 AM   #3
mehavey
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Which is why I asked if others have data points to add.

That the Lapua fell right in line with the Gov't case and the Commercial Win case gave me hope there might be reasonable linearity as long we're dealing w/ "common" brass. (Anyone have a Federal case to test that theory?)

As FrankenMauser has noted in the past, brass manufacturing is distributed all over the place.
http://thefiringline.com/forums/show...5&postcount=15
So I am curious as to how well the linearity holds up

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Old June 3, 2012, 09:11 AM   #4
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Alloy of the case? Is anybody doing precision reloading of anything but brass?
I doubt there is much handloading of anything but 70-30 Cartridge Brass, and there is not much difference in density if the mill sends along some 71-29.

Shape of the volume? I don't think the principle applies to anything but cartridges of different manufacture for the same chamber.


Once upon a time a moderately technical publication named The American Rifleman (not to be confused with the present collection of political bulletins and manufacturers' press releases of the same name) recommended that powder charge be reduced by one grain for every 11 grains of empty case weight. This back when it was generally true that military cases were heavier than commercial; which is the case in the example, but not always now.
17 grains more brass => 1 1/2 grains less powder by that rule of thumb.
How does that compute?
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Old June 3, 2012, 09:50 AM   #5
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Looking at all this, an "approximate" 10-to-1 ratio of case weight-to-powder weight (variation) might not be all that bad a starting point for medium-large military bottleneck calibers.

Interesting.....

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Old June 3, 2012, 10:19 AM   #6
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QuickLoad certainly thinks there's a direct, constant ratio, IIRC.

In the cartridge dimension section, if you give it the external dimensions and weight, it calculates volume.

Unless I'm backwards and you give it volume and it gives you weight.... I need a new copy.
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Old June 3, 2012, 10:42 AM   #7
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Quote:
... [QL] you give it volume and it gives you weight.
But external dimensions hide internal construction... hence the difference between the LC`81/WW weights/volumes.

If life (and women) were only that simple......)
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Old June 3, 2012, 10:52 AM   #8
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How close are QLs volume predictions between the 3 brands if you give it the dimensions?
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Old June 3, 2012, 12:18 PM   #9
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I will try to get you some federal data soon ...

PM me if I forget.
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Old June 3, 2012, 01:18 PM   #10
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As virtually all cartridge brass is the same metalurgy wise, for a given cubic millimeter or inch, they all weigh the same. And for a given chamber, a given amount of cartridge case brass weight takes up the same amount of space. Therefore, the room powder has to expand in equals chamber volume minus case brass volume. Heavier cases reduces that space more than lighter ones do.

I don't think judging case capacity by how much water it holds is all that accurate. Expecially when the case's real capacity is only reached when it's pressed hard against the chamber walls and bolt face at peak pressure. So heavier cases end up with less space inside than lighter ones.

While there may be some tiny difference in peak pressure from loaded rounds with different weights of water they'll hold, it's very small and insignificant. I think it's smaller than the typicaly round to round variations in equal charge weights, case neck tension and primer output that's also going to cause peak pressure and velocity spreads. Until one finds a way to keep these variables down to absolute zero, accurately measuring case capacity's influence on pressure and velocty won't be happening soon.

I've not seen enough case weight spread of 0.75% having any effect on accuracy through 1000 yards. And a 1% spread's good enough for use through 600 yards.
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Old June 3, 2012, 01:30 PM   #11
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Bart has some good points, but to help build the database / graph (for the nerds), here some of my old data:

All cases are fired (not resized). Some averaging of the numbers.

Lapua , tight chamber, dry 179.2, wet 233.8
Lapua , std chamber, dry 178, wet 232.2

LCM 65, dry 184.6 , wet 237.8
RA 60, dry 178.5, wet 232.6
WRA 66, dry 185.5, wet 239
LC 74 , dry 184.8 , wet 238.5

Winc 308, dry 171, wet 227.5
Fed , dry 181 , wet 236
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Old June 3, 2012, 03:29 PM   #12
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A little messier, but the correlation's still there...
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Old June 3, 2012, 04:51 PM   #13
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Mehavey, that' a good chart. Shows interesting data. The only thing that would put the icing on that cake of information would be case dimensions; pressure ring and shoulder diameter as well as case headspace (head to shoulder datum), neck diameter neck length and case overall length. One could compute the area of each conic section from a drawing to see how the total area of the case compared with its weight and water capacity. That would take an effort or three to do it.

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Old June 3, 2012, 05:24 PM   #14
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Old June 3, 2012, 05:26 PM   #15
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egad....






(I'll stick the newly "discovered" 10:1 rule.)
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Old June 3, 2012, 09:54 PM   #16
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To amplify some of what Bart said, the density of cartridge brass is 8.53 grams/cm³. IF the outside dimension of two cases are identical and IF the primer pockets and flash holes are identical, then the water volume difference will be 1/8.53 times greater for each same-unit-of-weight lighter that a case is. That's completely regardless of the internal geometry. It's just owing to the space the matter has to occupy unless you can cool the case to absolute zero (where Einstein has been proven correct that two different pieces of matter can then occupy the same space).

You will note the brass weight does not change during firing, but the external dimensions do. As QuickLOAD's instructions point out, in any gun firing at over about 30,000 psi, the fireformed volume rather than the as-sized volume winds up determining peak pressure, so it is that larger water volume you want to measure to figure peak pressures. However, fireforming to a given chamber also makes the outside dimensions from the pressure ring forward pretty close to the same, so at that point just dividing the weight difference by 8.53 is an even closer approximation of the water capacity difference.


Mehavey,

The reason you get the nice linearity is external brass dimensions seem to be pretty consistent in new brass. The head diameters are allowed to be 0.473" -0.008", and the military and commercial alike seem to aim on the low side, probably for feed reliability, and 0.467" is a pretty common sort of value.

I bought 500 pieces of Winchester brass about 5 years ago that weighed an average of 156.5 grains. The weight distribution showed the brass came off four different sets of tools (see histogram at the end). Old Winchester brass could be a good bit heavier, but modern Winchester brand .308 Winchester brass has the semi-balloon head (primer pocket inside is a bump up into the case space) designed by Winchester to add powder capacity for the 1992 Palma Match ammunition. They have since adopted it for their regular .308 brass.

For rounds firing over about 30,000 psi, the fireformed volume rather than the as-sized volume predicts pressure performance. But as I mentioned, if you fire the brass all in the same chamber and at similar pressure, you make it all pretty close to the same externally, so the difference in capacity predicted by the weight difference still applies. Among the brass I have, the weight goes from 156 grains (Winchester) to 186 grains (IMI Match). That 30 grain difference divided by 8.53 is just over 3.5 grains of water. I use .6 to .7 grains of powder per grain of water as a rough estimate of the difference the water capacity makes, so that's about 2 grains more powder in the Winchester than you would use with the same bullet in IMI Match to get same pressure performance.

All the other military type cases have much less difference in capacity from their commercial counterparts than the .308/7.62 do. In .223/5.56, the difference is essentially negligible in the vase majority of it. In .30-06 I've seen some Winchester as much as 10 grains lighter than some military brass (say, 185 grains vs. 195 grains), but Remington seems to match Lake City pretty closely. Some very old Winchester Western was as much as 215 grains. Nobody wastes brass like that anymore.

The only case with more spread than the various .308's is the .300 Win Mag. QuickLOAD's database finds them so different they actually list the different brands separately. The difference, from Remington (least capacity) to Norma (most capacity) is 7.5 grains.

Anyway, bottom line is to fire it all in the same chamber for your chart. Then you'll find the line stays very straight.

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Old June 3, 2012, 10:43 PM   #17
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More Data, but so what?
223/5.56 do not really vary much.
Not nearly as much as 308 in wt or %

Data is from: http://www.accurateshooter.com/cartridge-guides/223rem/
Attached Images
File Type: jpg Wt to Capacity.jpg (76.6 KB, 15 views)
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Old June 4, 2012, 05:58 AM   #18
Bart B.
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Marco Califo claims:
Quote:
More Data, but so what?
223/5.56 do not really vary much.
Not nearly as much as 308 in wt or %
True, but the ratio of water or powder weight to case weight's about the same, isn't it?
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Old June 4, 2012, 09:40 PM   #19
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I broke Marco's data into a graph looking for the expected inverse relationship between case weight and water capacity. I don't know if he fired all those cases in the same chamber before measuring water capacity or not? If so, I would have expected less scatter. If not, then the scatter looks kind of like Mehavey's when he started including data from cases not fired in the same chamber. There are, however, two exceptions, and those are the two cases that weigh 104 grains and 104.5 grains. They are outliers. The 104.5 grain case has more capacity than expected, and may have had a head nearer maximum diameter than is usual. The 104 grain case had less capacity than expected and may have had a smaller diameter head than usual, putting the extra brass somewhere inside the case instead. I'll have to do some geometry to see if that accounts for it. Otherwise, the black trend line shows where all the points would lie if the relationship were exactly regular.

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Old June 4, 2012, 10:48 PM   #20
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Was not my own data

Was not my own data. I added the source above. I guess I am not as interested in the statistical side of this, as you guys are.
What I found useful was that 1/10th out of 30.0 is 1/300, or 1/3 0f one percent. Therefore, no need to use different loads for commercial vs. mil-spec brass in 223. In 308 there is quite a bit more variation, and I do load them differently. I believe there are too many variables in play to say x weight of brass = y of water, with any meaningful consistency.
The only practical info I see here is that, yes, 308's vary. So, I sort them into groups for reloading, and follow the recommendations, like Sierra's: "If military brass is used for reloading, the charges shown should be reduced by one to two grains."

Here is how they measured:

* Average case weight of sized, deprimed, once fired brass (except new Lapua lot). Weight is average of sample lot.
** Case capacity of three sample cases that weighed closest to the average weight of that brand. All cases were full-length sized and trimmed to 1.750″. A spent primer was inserted in the primer pocket, the case weighed. Then the cases filled with distilled water to the top of the case mouth and weighed again.
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Old June 5, 2012, 05:10 AM   #21
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I'm convinced that several cases, all the same exact weight and length from the same lot will hold different amounts of water when filled to the case mouth. As they're all not perfectly round, those more oval (egg?) shaped than others will hold less water. My reasoning's based on the fact that if you make their shape more oval, their capacity's gets less. As the major axis gets bigger, the minor axis gets smaller. Make the major axis big enough and the small one goes to zero.

The formula for the area of an oval is pi*a*b where a = half length of major axis and b = half length of minor axis.

As the actual amount they vary is small, it may be noticed if the water's weighed accurate enough.

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Old June 6, 2012, 01:35 PM   #22
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As I said earlier, IF outside dimensions are identical, capacities will correspond to weight; of not, all bets are off.

I noticed the data Marco has is for resized brass, and not as-fired brass. Also, the article had no statement as to whether they were fired in the same gun or not. Given that they resized first, I'm guessing not. As we all know, a resizing die will resize cases from different chambers with different degrees of success, as the spring-back isn't the same when you start with idifferent sizes. Even cases with different load histories can vary several thousandths. I still expect those considerations account for most of the scatter in the plot except for the last two points.
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