Do any of you guys use this method for case sorting?

[DougMH]

All,

Because I don't have any confidence in any instrument that measures neck tension (not even the K & M with force measurement. I've seen too many spreadsheets that indicate it doesn't work so well), I've come up with my own method of reducing my muzzle velocity (MV) standard deviation (SD).

I’m going to sort my cases by MV. Provided you've done meticulous preparation otherwise, it should be the perfect way (but I think on fire formed cases only).

You have a 100 round bullet case and you number left to right, top to bottom.

1 2 3 4 5 6 7 8 9 10
11 etc
.
.
.
100

You have an Excel Spreadsheet to record the corresponding 100 MVs.

You fire Round1, store Case1 and record MV1. You repeat this 100 times.

Only 11 in the example below but it makes the point. You then find the mean, median, mode… with mode probably being the most important. If the mode is greater than the mean, then take out from lowest MV toward highest. If the mode is less than the mean, take out from highest to lowest. You just keep removing MVs and let Excel recalculate until you have a SD <= 3. Here’s an Excel example

11 Rounds. After all 11 are fired, I have an SD of 17+

2750
2744
2743
2760
2800
2744
2743
2739
2745
2747
2748
Mean 2751.182
Median 2745
Mode 2744
StdDev 17.06938
Max 2800
Min 2739

Since the Mode is < the Median, I start thowing out from the high side. I thow out 2800 (case #5) resulting in …

2750
2744
2743
2760

2744
2743
2739
2745
2747
2748
Mean 2746.3
Median 2744.5
Mode 2744
StdDev 5.696978
Max 2760
Min 2739

I then throw out 2760 (case #4) resulting in…

2750
2744
2743


2744
2743
2739
2745
2747
2748
Mean 2744.778
Median 2744
Mode 2744
StdDev 3.231787
Max 2750
Min 2739


Then I throw out 2750 (case # 2)…


2744
2743


2744
2743
2739
2745
2747
2748
Mean 2744.125
Median 2744
Mode 2744
StdDev 2.748376
Max 2748
Min 2739

… and that puts me under 3.00 for SD. Now for my upcoming match, I can use all the remaining cases. If all I do is decap, clean, re-neck size, nothing significant should change. I could probably even re-anneal them as long as I was very consistent about it.

Of course this requires extensive testing to prove to a statistical certainty.

Comments solicited?

 

[ReedG]

My opinion is that the next time you fire this same brass you will get a different set of numbers. I am not aware of any method of sorting that will assure repeatable results. In the quest for consistent, repeatable results, my experience shows that the cases must be annealed every firing.

I do have to say, though, that "statistical certainty" is not a world I live in.

 

[jlow]

I think the best way to prove how well this work is to do the test that ReedG suggested. i.e. actually fire and test all of the above 11 pieces (a lot more would be much better) and see if there is any correlation between the MV between the two. You could actually plot the MV from the first firing against the second and assuming everything is perfect, you should get a straight line with all the points falling on it. In addition, you can use those points to calculate the statistical value of correlation coefficient which will give you a value with 1 being perfect and 0 being absolutely no correlation.

The potential problem as I see it is that the assumption here is that all the variation is due to the brass. Since you are going for very low SD, even slight changes which can creep in the process of cleaning, re-neck size, characteristics of you bullet, powder weight, condition of the barrel, primer consistency etc can throw you off. Would however be very interested in what you find.

 

[Tozguy]

Very interesting question. I agree with Reed and jlow that it is more a question of correlation between a case number and its rank in a velocity spread than of manipulating standard deviation calculations. It would be necessary to shoot a certain number of cases a certain number of times to see how closely each case maintains its rank in the velocity spread from cycle to cycle. The trick would be to keep everything else equal. I'll be looking it up but I believe that correlation calculations have a certain tolerance for invisible variations.

 

[6BRinNZ]

Doug - yes and no is my answer to your question. I have a slight variation of your approach. No - I don't use MV to sort cases. With load development I do work to the keep it under 20fps ES rule for 1000 yard shooting but once I come inside of that and I have a load that is consistently accurate, I drop into the following;

Cases that don't hold x ring in club days get put aside.

All the cases that hold x ring get used for regional comps. My approach is relevant to F-class which is for score not group.

My theory is that SD/ES are only indicators of what will actually happen on paper, not necessarily absolutes of what result I will see on paper. Interestingly shooters argue about which is more important SD or ES. I think this is quite relevant to distances shot at as well.

I believe this site posted an article by the Eubers (?) some time ago where they marked each case and always shot it in order. When shots started dropping out of the group that case would be replaced. This article was the basis for my starting point.

 

[watercam]

I think that if you want to sort this way, I for one will not stop you...

 

[DougMH]

I definitely should have included ES in my spreadsheet. Also, I should take from both high and low in some proportion so as to narrow both ES and SD.

I'll have time to do this at length starting June 15th or so.

I'll post results good or bad.

 

[SpencerC]

What chronograph are you using and what is it's accuracy range? I'm using a cheaper Chrony F-1 and under perfect conditions it is only accurate to 99.5% (so it says..) now throw in lighting conditions, muzzle distance variations etc.. and your % accuracy goes down the hole. If I was really picky about sorting brass I would sort by volume with distilled water and the same spent primer, but I'm not, so I only weight sort. To me, weight sorting is as far as I can go before the limits of all of my equipment come into play, not to mention the nut behind the trigger and his wind reading skills or lack there-of.

 

[GermanS1]

Doug, no disrespect intended, but I suppose that might be a god method if the match was one in which you were measuring MV instead of group size or score.

There's more to consistent MV than the brass, as you know and there's more to good brass than consistent MV.

One of the biggest flaws in brass is uneven case wall thickness (not simply neck wall thickness). An uneven case might have equal volume to its mates and give you MV right at the mean, but it will throw the shot out of a group. A case like that develops a strong banana shape and induces irregular harmonics in the barrel. Here's a short item for you on that topic: http://riflemansjournal.blogspot.com/2009/07/reloading-audette-neco-case-checker.html

When testing equipment and loads, try to maximize the result that is measured in your match. For me, that's a 20 shot score, prone, with iron sights, so that's how I test. For others it might be 5 shot group size with a scope from a rest - that's how they should test and measure. You can spend a lot of times chasing the non-essential.