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Topic: Alpha 6 BRA - CC Vs Weight (Read 167 times) previous topic - next topic

Alpha 6 BRA - CC Vs Weight

400 Virgin OCD Cases:

Cases were initially sorted by weight into 0.1 grain increments. The selected ranges varied from 131.0 to 131.9 grains of dry brass weight. Twelve cases weighed in outside that range and were culled from further measurements. The weight sort produced a much flatter bell curve than I have seen with Alpha brass in the past. The 131.0 gr and 131.9 gr groups each contained 20 or so cases, while the bell peak was at 131.6 gr with 67 cases total. A weight curve of this shape is a bit frustrating because it frequently indicates a broad spectrum of case capacity which leads to significant vertical at distance unless a selection process is conducted.

To delineate that concern, case capacities were measured using a Sartorious scale delivering 0.002 gr +/- accuracy. A solution of one small drop of Dawn dish soap in one quart of water was used with a fine tipped dose syringe for delivery or removal of liquid to cases being measured. A 21st Century primer cup cap was used for each case with the plug being tissue paper dried before transfer between cases. Tare weight for this cap was 270.030 grains both before and after all measurements. Three cases from each weight group were arbitrarily selected and water capacity measured. Total fill was determined using a bright over head light to insure a flat fluid level at the top of each case. A totally flat miniscus can be obtained with the proper soap solution. A fine tipped syringe allows removal of any excess solution at the top of the case with aspiration. Many times you have to add or subtract a small amount of fluid to attain a level fill.

Average CC of the 131.0 gr weighted cases was 37.955 gr of H20 and the same measurement of the 131.9 gr weighted cases was 37.871. The variance rounded to 0.1 gr of H20 capacity between the lightest to heaviest cases measured. Average case capacities in the weight groups between the two extremes remained within the ES of the 131.0 and 131.9 groups.

To determine variance within groups, 10 cases from the 131.0gr group and 10 cases from the 131.9 gr group were arbitrarily selected and measured for case capacity. The 10 case average CC for the 131.0 gr group was 37.933 gr, while the 10 case average for the 131.9 gr group was 37.842 gr. of H20. ES in the 131.0gr group was approximately 0.1 gr and ES in the the 131.9 gr group was approximately 0.08 gr H20.

The predictive validity of case dry weight to actual case capacity has been a subject of many debates over the years. Measured numbers have been presented by many reloaders that argue both for and against the use of case weight to select for uniform case capacity.

There are several take-aways from the data reported here.
Number one - Alpha brass is amazingly consistent
Two - case weight may give some insight into case capacity but not always as much as we would like to believe. In prior case capacity measurements that I have done, there has always been an inverse relationship between case weight and case capacity. While one trends up the other usually trends down. I’ve usually found close to 0.1 gr to 0.1 gr change in weight versus capacity. That was not so with this particular  batch of Alpha brass. Over a 0.9 gr variance in dry weight there was only an approximate 0.1 gr change in case capacity. Is that a fluke or is it further proof of Alpha’s quality control? I don’t know.
Third - the recorded range of dry weights in this batch of brass would normally have prompted me to separate this brass into at least 4 different groups for competition reloading.  These findings made me very comfortable dividing the total 388 cases in to just two groups making my reloading life much easier. So take-away number 3 is - there is no substitute for at least sampling case capacity in the competition brass you are going to sort. 😊
Number 4 - Alpha brass pays for itself.

As mentioned, these measurements come from virgin brass. After fire forming and uniforming case lengths, I will go back and repeat these measurements for comparison.

Disclaimer: please do not assume a 0.9 gr weight variance will produce this degree of consistency in case volume in a different brand of brass or even in a different batch of Alpha brass. This is simply a report on what I’ve found with my particular batch of brass.

Re: Alpha 6 BRA - CC Vs Weight

Reply #1
You must have been really bored!!!

Nice job.  I know we have talked about this very subject.
You never will know till you actually do it.
I'd send this info to Alpha brass (Tom)
Makes me feel better about my Alpha 6bra cases.

Re: Alpha 6 BRA - CC Vs Weight

Reply #2
a couple things that impresses me...
one is the alpha brass...
second is your dedication to being the best you can be....
Grant

Re: Alpha 6 BRA - CC Vs Weight

Reply #3
a couple things that impresses me...
one is the alpha brass...
second is your dedication to being the best you can be....

It’s only been a few years that I’ve been able to compare Alpha versus Lapua brass. The Alpha keeps coming out on top - consistently. I have a lot of respect for their quality control and I sure hope they don’t stray away from that effort.

Re: Alpha 6 BRA - CC Vs Weight

Reply #4
You must have been really bored!!!

Nice job.  I know we have talked about this very subject.
You never will know till you actually do it.
I'd send this info to Alpha brass (Tom)
Makes me feel better about my Alpha 6bra cases.

Maybe you could do some measurements on your batch for a comparison. Or you could send me a 100 of them to check out. Might take me a couple years and a few cycles before I get them back to you. 😊

Re: Alpha 6 BRA - CC Vs Weight

Reply #5
I did weigh mine when I received them and weighed them again
after turning them down. The "groups" or difference between them
remained almost the same. Had a few culls.
I had wondered if the weight difference could be attributed to
excess weight in the neck area. But since the necks are very even to begin with
and not much variance in neck thickness, that proved not to be a concern.
Also, not much weight is taken off the necks compared to the rest of the total weight
of the case, so it didn't change the out come.

Your next test should be that you take the lowest and highest weight cases
load some up and see if it translates to a POI difference at distance.
Trying to keep all else equal.
 



Re: Alpha 6 BRA - CC Vs Weight

Reply #6
“Your next test should be that you take the lowest and highest weight cases
load some up and see if it translates to a POI difference at distance.
Trying to keep all else equal.“

Agreed however, if you run the numbers in QuikLoad, a 0.1 grain difference in case capacity generates a whole 2 fps change in velocity. I think that number is going to get buried in the other internal ballistic variables.

Re: Alpha 6 BRA - CC Vs Weight

Reply #7
Agreed however, if you run the numbers in QuikLoad, a 0.1 grain difference in case capacity generates a whole 2 fps change in velocity. I think that number is going to get buried in the other internal ballistic variables.

I'd crunched the same numbers in QL as well Jerry, in 6.5 with a proportional CC difference; until I can reliably see single digit ES in real life, 2 fps is buried in the noise.

Ranger 188 mentioned something I'd though about; case weight differences showing up in the neck, could that affect neck tension?

Anyway, it looks as if you've really crossed the T's and dotted the i's as far a CC measurement goes; that's commendable. It's not easy to do. If I can measure dry brass weight for a single piece, and get the same value 3 times in a row, but I can't get the same water filled weight even twice is a row, that's discouraging to me.
Chris,
Retiring end of Q1-2021 :)

Re: Alpha 6 BRA - CC Vs Weight

Reply #8
“Ranger 188 mentioned something I'd though about; case weight differences showing up in the neck, could that affect neck tension?“

Yes that would affect neck tension but if you turn the necks to an even thickness that problem should go away.

I should mention that the 388 cases that I measured DID have their necks turned. So the cases were virgin in that they had not been fired, but not virgin from the standpoint of the necks. As Greg said, the Alpha necks are crazy consistent right out of the box.

Re: Alpha 6 BRA - CC Vs Weight

Reply #9
“ but I can't get the same water filled weight even twice is a row, that's discouraging to me.“

You add a lot of variables to the measurement when adding a liquid. I don’t think it’s rational to expect the same result each time. Use the same technique with each case as closely as you can, and measure multiple cases to buffer the variations.

Re: Alpha 6 BRA - CC Vs Weight

Reply #10
Bottom line is, Alpha OCD cases are about as good as your going to get.

Re: Alpha 6 BRA - CC Vs Weight

Reply #11
“ but I can't get the same water filled weight even twice is a row, that's discouraging to me.“

You add a lot of variables to the measurement when adding a liquid. I don’t think it’s rational to expect the same result each time. Use the same technique with each case as closely as you can, and measure multiple cases to buffer the variations.

That's true to a point but, I get less interested in two variables I suspect, or hope, might be correlated when one of them may be measured with a great deal of precision and repeatability, worthy of a Satorious scale, while the other is loosey-goosey, close-enough, postal scale grade. That's just me though I'm still in awe and appreciative of the work you did.
Chris,
Retiring end of Q1-2021 :)

Re: Alpha 6 BRA - CC Vs Weight

Reply #12
Chris did you arrive at a correlation factor between case weight & volume?My thought is that it may be more linear than one would guess
11X Grandfather
Part time Savagesmith

Re: Alpha 6 BRA - CC Vs Weight

Reply #13
That's true to a point but, I get less interested in two variables I suspect, or hope, might be correlated when one of them may be measured with a great deal of precision and repeatability, worthy of a Satorious scale, while the other is loosey-goosey, close-enough, postal scale grade. That's just me though I'm still in awe and appreciative of the work you did.

 Cases were weighed with a FX-120 (accuracy @ +/- 0.02). Cases were grouped into 0.1 increments using a 0.05 +/- range). Accurate as I can get them into 0.1 gr increments. No postal scale. 😊

 

Re: Alpha 6 BRA - CC Vs Weight

Reply #14
Chris did you arrive at a correlation factor between case weight & volume?My thought is that it may be more linear than one would guess

I threw in the towel for now Randy, I didn't trust the consistency of the results I was getting with case capacity measurements. Jerry has inspired me to revisit it though, need to work on my technique. 
Chris,
Retiring end of Q1-2021 :)