How much case weight variation is significant?

[Mulligan]

In testing, how much variation in case weight is detectable at 600 or a 1000 yards in a BR/Dasher size case?
Or a better question, what is the acceptable tolerance per bin?
I have tested it a little and I think I can see .5gr and know I can 1.0gr variation in round robin case weight tests.
CW

 

[rkittine]

I do not have enough experience to answer your question, but I figure if I prep my brass all the same, trim to length (all new boxes of brass get trimmed to the shortest - assuming in specification - length case in the box) , reaming primer pockets, full cleaning etc. and there is a variation in case weight, then there is probably a variation in wall thickness and hence a possible difference in internal capacity after sizing that would vary pressure from round to round. So after prepping all cases the same, I weight them and sort them into batches. Since I only use Lapua Brass and have not seen great fluctuations in prepped case weight, if I have say 3 or 4 batches out of 100 new pieces of brass, I can easily sort a batch of Match Brass.

I am doing this with 6PPC, 6BR, 6.5x47 L and .308 Palma Cases.

I try to do internal volume capacity too, but I get enough variations in my own results on the same case, to have somewhat given up on that.

Bob

 

[atblis]

Not a direct comparison but there's an AMU write up somewhere backed by testing where they determined what case weight variation mattered for their purposes. I'll see if I can dig it up.

 

[Mulligan]

I do not have enough experience to answer your question, but I figure if I prep my brass all the same, trim to length (all new boxes of brass get trimmed to the shortest - assuming in specification - length case in the box) , reaming primer pockets, full cleaning etc. and there is a variation in case weight, then there is probably a variation in wall thickness and hence a possible difference in internal capacity after sizing that would vary pressure from round to round. So after prepping all cases the same, I weight them and sort them into batches. Since I only use Lapua Brass and have not seen great fluctuations in prepped case weight, if I have say 3 or 4 batches out of 100 new pieces of brass, I can easily sort a batch of Match Brass.

I am doing this with 6PPC, 6BR, 6.5x47 L and .308 Palma Cases.

I try to do internal volume capacity too, but I get enough variations in my own results on the same case, to have somewhat given up on that.

Bob

That is funny.
I tried the same thing, and after much frustration I followed some advice I received on this forum and used the average of 5 tests of internal volume on each case using ball powder. I plotted the project and weighted sorted each piece of brass and volume sorted with water........ none of the three had any correlation whatsoever when I velocity tested the brass. This was with Lapua 6 BR brass made into dasher brass. For Lapua 6 BR brass, I use the target to tell me how to sort.
Norma 6.5x284 brass that I have weight tested, there has been a direct correlation to shot placement on target.
CW

 

[Mulligan]

This is the weight sorted Norma 6.5x284 necked up to straight 284 I mentioned in the post above. I do not remember the variation in weights, but is was the 5 heaviest and 5 lightest in a box of 100.
On some brass there is a correlation, not sure it always is this way.
CW

 

[rkittine]

Is the upper group from the heaviest or the lightest? I would think that the heavier brass would have thicker walls, more pressure with the same load and therefore less vertical drop.

RPK

 

[Mulligan]

Is the upper group from the heaviest or the lightest? I would think that the heavier brass would have thicker walls, more pressure with the same load and therefore less vertical drop.

RPK

Yes, heavier brass is on the top.
CW

 

[Mulligan]

shot this group yesterday. 6 Dasher heaviest 5 pieces are red, lightest 5 are yellow.
600 yards
CW

 

[dgeesaman]

That is funny.
I tried the same thing, and after much frustration I followed some advice I received on this forum and used the average of 5 tests of internal volume on each case using ball powder. I plotted the project and weighted sorted each piece of brass and volume sorted with water........ none of the three had any correlation whatsoever when I velocity tested the brass. This was with Lapua 6 BR brass made into dasher brass. For Lapua 6 BR brass, I use the target to tell me how to sort.
Norma 6.5x284 brass that I have weight tested, there has been a direct correlation to shot placement on target.
CW

Ditto, for me velocity testing cases and plotting velocity vs volume and weight. It could be my velocities aren’t tight enough to show a correlation.

David

 

[Zero333]

Dasher size case for competition I'd group them in 0.5 gr increments.

 

[mikecr]

There is no rational guess about it.

 

[Ned Ludd]

With a few qualifications, case weight is inversely to proportional to case volume. The relationship is not always "perfect", meaning you cannot rely on weight differences between individual cases to provide exact differences in internal volume. Nonetheless, weight-sorting cases will generally provide significantly more uniform internal volume than doing nothing at all.

As to how much variance is significant, that is a more difficult question to answer in an all-encompassing manner. The simplest answer is that if case volume variance reaches a certain fraction of the total internal volume, it will start to show up in velocity measurements. As to exactly what that minimum value is, it will likely depend on the total case volume, and perhaps even the shape of the cartridge. I've never tried to calulate the minimum weight variance necessary to cause a detectable change in velocity for a given cartridge, it much easier just to load highest/lowest case weights from a Lot of brass, load them identically, and determine empirically whether your chronograph can detect the difference. If not, sorting them by weight probably won't do much. In a relatively small case such as the .223 Rem, the difference in velocity between the highest/lowest case weights can be as mush as 25 to 30 fps with Lapua brass.

If you happen to have such a Lot of brass that exhibits a noticeable velocity difference between heaviest/lightest case weights, it wouldn't be too difficult to select something like 10 each of the heaviest/lightest cases, weigh them, load them up identically and determine velocity with each, then determine actual water volume afterward. Armed with that information, as well as the density of brass, you could probably fudge up some numbers with respect to how much volume or case weight variance was necessary to be significant for that specific cartridge and brand of bass.

 

[David Christian]

I wish I would have learned this lesson several years ago. Buy whatever quantity of brass that you need from the same lot and sort them by weight. For example, big matches for me need almost 600 rounds. I bought 700 brand new pieces of brass, weight sorted all of them and then formed 6 x 100 new lots that are .3 grains in weight a part, culling some from the top and bottom. Like @Ned Ludd said, not a 100% correlation, but sorting this way will at least tighten up your spreads.

 

[INTJ]

I have no issue getting ES under 11 FPS for five shot groups with cases sorted into 1 grain lots. That's with both the 300 WSM and the 6 BRA. It's questionable if we even need that low of ES to shoot screamer groups at 600 and 1000 yds.

I suppose the ultimate way to sort cases would be each time after they are all sized and trimmed, to insert plugs that seals the all primer holes exactly the same then fill with water and sort by internal volume. That assumes that we are super consistent in determining when a case is full. Then we remove the plugs, dry the cases, and finish loading.

If that was done would ES be reduced? Maybe. Would it make any difference in a long range BR target? Doubtful.

It's not uncommon at all to have higher ES loads produce smaller groups than lower ES loads.

So how much variation in case volume will show up on the target? Has any one done such a test? Could such a test be done without being overwhelmed by conditions?

 

[bcarico]

Lots of opinions that a lot of stuff like this doesn’t matter. I can bet you the guys winning 1000yd benchrest week after week are doing a lot of stuff on the loading bench others say don’t matter.

 

[group therapy]

Lots of opinions that a lot of stuff like this doesn’t matter. I can bet you the guys winning 1000yd benchrest week after week are doing a lot of stuff on the loading bench others say don’t matter.

I was thinking that myself!!!!!! We eliminate variables (well, br guys do) and that’s one variable we can put in its place

 

[INTJ]

Lots of opinions that a lot of stuff like this doesn’t matter. I can bet you the guys winning 1000yd benchrest week after week are doing a lot of stuff on the loading bench others say don’t matter.

We also stop doing things that don't make a difference. It takes me a long time to twice measure and spin bullets. That makes a difference. So does weighing primers and seating them the precisely the same amount of crush. So does measuring every case after sizing and every bullet after seating.

I don't have time to do things that don't enhance accuracy.

And with all that, we never see the same guys winning all the time. There is significant variation in who wins what in each match. That is because in LR BR, conditions almost always overwhelm our careful load prep.

On an average day half of what we do doesn't make a difference. It's only that 10% of the time when conditions are good that the anal stuff matters. And on those days we definitely see who is the most well tuned.

 

[David Christian]

I have no issue getting ES under 11 FPS for five shot groups with cases sorted into 1 grain lots. That's with both the 300 WSM and the 6 BRA. It's questionable if we even need that low of ES to shoot screamer groups at 600 and 1000 yds.

I suppose the ultimate way to sort cases would be each time after they are all sized and trimmed, to insert plugs that seals the all primer holes exactly the same then fill with water and sort by internal volume. That assumes that we are super consistent in determining when a case is full. Then we remove the plugs, dry the cases, and finish loading.

If that was done would ES be reduced? Maybe. Would it make any difference in a long range BR target? Doubtful.

It's not uncommon at all to have higher ES loads produce smaller groups than lower ES loads.

So how much variation in case volume will show up on the target? Has annoying done such a test? Could such a test be done without being overwhelmed by conditions?

@INTJ , I don’t shoot BR, but many of the same reloading concepts help me in my 20 shot strings for F-Open competitions. When 1-2 points separates placing 1st and 10th over a 120 shot competition, I am sure to balance time invested versus benefit gained with each micro loading technique.

I have run such water weight testing using the same 100 pieces of brass tracked individually over 5 firings (weighed and water weight tested each time) and have found the same loose correlation as stated above. No need for me to repeat such a mind numbing exercise. I will go with the underlying end result and simply sort my brass by weight as a suitable and best effort solution. Time invested versus end result benefit.

As an aside, Assuming proper node identification, I will take the low ES group over high all day long, especially out past 600 yards. At 1,000 yards for my particular bullet and chambering the difference in an ES of 20 and an ES of 30 produces an extra 2” of vertical. Could that not create one or two more vertical 9’s?

 

[INTJ]

@INTJ , I don’t shoot BR, but many of the same reloading concepts help me in my 20 shot strings for F-Open competitions. When 1-2 points separates placing 1st and 10th over a 120 shot competition, I am sure to balance time invested versus benefit gained with each micro loading technique.

I have run such water weight testing using the same 100 pieces of brass tracked individually over 5 firings (weighed and water weight tested each time) and have found the same loose correlation as stated above. No need for me to repeat such a mind numbing exercise. I will go with the underlying end result and simply sort my brass by weight as a suitable and best effort solution. Time invested versus end result benefit.

As an aside, Assuming proper node identification, I will take the low ES group over high all day long, especially out past 600 yards. At 1,000 yards for my particular bullet and chambering the difference in an ES of 20 and an ES of 30 produces an extra 2” of vertical. Could that not create one or two more vertical 9’s?

I think you F-Open shooters tune as well as LR BR shooters. @Ned Ludd has also talked of such tests with water volume and I think doing weight sorting is plenty good enough for extreme accuracy.

I don't use a chronograph when tuning at 600 or 1000 yds. It's only about group sizes. At 200 yds I do use a chronograph but as long as the ES are under 20 FPS I will pick group size over ES. There does seem to me merit in the theory of "positive compensation". I am usually in the teens or below, and if I am over 20 fps ES then the load just isn't right and I keep working.

I guess I should use a chronograph when I get to test at 600 and 1000 yds, usually the day before the match. It would be interesting to see if my short range assumptions hold true at long range.

 

[Ned Ludd]

...So how much variation in case volume will show up on the target? Has annoying done such a test? Could such a test be done without being overwhelmed by conditions?

This is what I alluded to. It can be done with relatively minimal effort. First you need to sort your cases by weight, water volume, or (preferably) both, so that you can select a set (each) of the highest/lowest weight (or volume) cases. Then you load them identically and measure velocity to determine whether you can actually measure a velocity difference between the two sets. It's basically a single range trip. If you wish, you could also add a "medium" group in with the high/low, but it might not add much to the initial experiment. The key is to test the two extremes frst to determine whether there is any measurable difference. If there is not, there is no reason to go any further.

However, if the answer to the question of whether there is a velocity difference between the high/low sets of cases is is "yes", simply determine the velocity difference and the case volume/weight difference for the two sets by subtraction. Then divide measured velocity difference by the difference in weight (or volume) for the same sets of cases. This will give you an average velocity variance on a per weight (or per volume) basis. You can then set whatever limit you want as far as how much case weight (or volume) variance you are willing to accept when sorting cases in terms of how much it will affect velocity.

Again, these approaches are relatively simple and will most likely suffer from sampling error issues if you try to apply them to individual or very small numbers of cases. However, this is a situation where it is best to let the statistics work in your favor, which is really the intent when we "sort" anything. Regardless of the presence of a few individual outliers within a group, it is usually possible to prepare sorting groups such that the the sorted group average will be much tighter than doing nothing at all. As a starting point to determine the usefulness of any sorting approach, I always like to keep things as simple as possible initially, then expand the scope later if I feel it is warranted.