Revisiting: sorting cases by weight

[ntsqd]

I've been searching here and have found quite a bit on the topic, but most of those threads are at least a couple years old if not older. I'm curious if this is considered "put to bed" or recent experience has lead to revisions of thought.

I see there's a school of thought that says measure case volume, case weight is useless.

And I see the school of thought that says case weight is a reasonable indicator of case volume.

My own thinking, right or wrong, is that if you filled the chamber with melted-down case brass, trimmed to case length, and weighed that, then weighed a case fired in that chamber that, less the elastic return of the fired case, the difference in weights is the case volume. This assumes that the brass density is the same. I would assume that they are not the exact same density, but that the delta is small enough that in the volumes under consideration that statistically it is noise.
So my conclusion is that weighing cases is an indirect measure of case volume that within mfg lots is consistent enough for our sorting purposes, but that considering weights of different mfg's cases or even cases of the same mfg, but from different lots is an error.

The question that lead to this search is: What tolerance is allowable for "like" weight cases? 0%? 1%? 10%? i.e. with cases in the ~116 grain weight range (.224 Valkyrie) what is an acceptable sorting increment? Sort by 1.0 grains? Sort by 0.1 grains? Sort by 0.01 grains?

 

[ntsqd]

I guess it was put to bed then?

What about weight tolerance?

 

[ShtrRdy]

Not put to bed in my mind. I'm hoping to try an experiment with some Lapua .308 brass I bought recently. I weighed the 100 pieces of brass in that box and had a total spread of 3.3 grain. I thought this was a lot for Lapua because the two boxes of 6mm Creedmoor I bought only had a weight spread of 1.2 grain with 165 of the 202 pieces being within 0.5 grain.

I contacted Lapua and they said the volume of the case is what they strive for. So I'm going to take a few of the lightest and a few of the heaviest pieces of .308 brass and fire them over the chrono. ( magnetospeed ). I'm going to do this twice so that I have twice fired brass and then measure the water weight to get an idea of the case volume.

 

[six pack]

Having a bit of statistical analysis exposure over the last 40+ years, I would bet my lunch money that the difference between cases manufactured by the same company but of different lots and sorted by tighter weight differences would be "noise", to borrow your word.

For example, two different lots of bulk XYZ brass, weight sorted by .2 or .3g instead of .5 to 1.0g would likely produce groups of cases that are as close or closer in volume. Potential exceptions being production dates that are several years apart or produced at different facilities but having the same head stamps.

Bottom line, I think one would be fine going to the LGS or Midway, or whoever and buying multiple bags/boxes of the same brand of bulk brass that may or may not be of different lots. Beyond that is overthought and maybe even a little OCD. Just my occasionally humble opinion but I'm open to being proved wrong. ;-)

 

[BLC3128]

I've been searching here and have found quite a bit on the topic, but most of those threads are at least a couple years old if not older. I'm curious if this is considered "put to bed" or recent experience has lead to revisions of thought.

I see there's a school of thought that says measure case volume, case weight is useless.

And I see the school of thought that says case weight is a reasonable indicator of case volume.

My own thinking, right or wrong, is that if you filled the chamber with melted-down case brass, trimmed to case length, and weighed that, then weighed a case fired in that chamber that, less the elastic return of the fired case, the difference in weights is the case volume. This assumes that the brass density is the same. I would assume that they are not the exact same density, but that the delta is small enough that in the volumes under consideration that statistically it is noise.
So my conclusion is that weighing cases is an indirect measure of case volume that within mfg lots is consistent enough for our sorting purposes, but that considering weights of different mfg's cases or even cases of the same mfg, but from different lots is an error.

The question that lead to this search is: What tolerance is allowable for "like" weight cases? 0%? 1%? 10%? i.e. with cases in the ~116 grain weight range (.224 Valkyrie) what is an acceptable sorting increment? Sort by 1.0 grains? Sort by 0.1 grains? Sort by 0.01 grains?

I'm from the school of weighing your brass. My method is to weigh 10 random cases, find the average and accept everything within a 0.5% range.*When calculating the AVG, in the event that I pick a noticeably heavier or lighter case, I set it aside and pick another.* Just last night out of 98 pieces of Norma Brass for my 300 WBY, I had to set aside 4 which didn't make the cut. They will be used for fouling shots.

 

[ntsqd]

My thinking behind the case lot constraint was that there was potential, be it true or not, for the brass density to vary more lot to lot than within a lot. I could be wrong there too.

With a ±0.5% tolerance in the .224 Valkyrie cases in question that means accept from 155.4gr to 116.2gr. OK, I like that a lot better than trying to find enough that all weigh exactly the same.

 

[Opa-lopa]

90+% of the volume/weight delta case/case within the same case lot is changes in the thickness of the case base, wall thickness differences account for the rest. My data says commercial production weight variation is about 1.6g for 30-06 case weighting about 200g, so less than 1%. If you put that tiny variance into Quickload or another interior Ballistics code, I think you'd be astonished how small an effect there is on MV.

 

[Danny Butler]

You are probably already doing this..... I uniform the primer pockets and bevel the flash holes... then trim all my new brass to exact length AND do not chamfer or bevel the case necks... then I weigh them. Makes a lots of difference for me..

 

[Zymurgist]

Not put to bed in my mind. I'm hoping to try an experiment with some Lapua .308 brass I bought recently. I weighed the 100 pieces of brass in that box and had a total spread of 3.3 grain. I thought this was a lot for Lapua because the two boxes of 6mm Creedmoor I bought only had a weight spread of 1.2 grain with 165 of the 202 pieces being within 0.5 grain.

I contacted Lapua and they said the volume of the case is what they strive for. So I'm going to take a few of the lightest and a few of the heaviest pieces of .308 brass and fire them over the chrono. ( magnetospeed ). I'm going to do this twice so that I have twice fired brass and then measure the water weight to get an idea of the case volume.

I'm not so sure that case weight variance is a good indicator of case volume variance as each case can be a little heavier or lighter in different parts of the case.

Not put to bed in my mind. I'm hoping to try an experiment with some Lapua .308 brass I bought recently. I weighed the 100 pieces of brass in that box and had a total spread of 3.3 grain. I thought this was a lot for Lapua because the two boxes of 6mm Creedmoor I bought only had a weight spread of 1.2 grain with 165 of the 202 pieces being within 0.5 grain.

I contacted Lapua and they said the volume of the case is what they strive for. So I'm going to take a few of the lightest and a few of the heaviest pieces of .308 brass and fire them over the chrono. ( magnetospeed ). I'm going to do this twice so that I have twice fired brass and then measure the water weight to get an idea of the case volume.

Your 3.3gr. spread is less than +/-1% with a SD of <0.55gr. I'd say that's pretty good for a mass produced case. I'm not so sure that case weight variance is a good indicator of case volume variance as each case can be a little heavier or lighter in different places of the case which my or may not affect the case's volume.

 

[Knightvision]

I buy the best cases I can find and weigh them after I check length and primer pockets and I weigh everyone and out of 10 there will be at least one that I reject, light or heavy its gone. I don't care what the brand or how much you spend I find discrepancies and they go in the 0file. The only ones I don't even bother with any longer is Remington, just way off the scale and not worth the time.

 

[Zymurgist]

Not put to bed in my mind. I'm hoping to try an experiment with some Lapua .308 brass I bought recently. I weighed the 100 pieces of brass in that box and had a total spread of 3.3 grain. I thought this was a lot for Lapua because the two boxes of 6mm Creedmoor I bought only had a weight spread of 1.2 grain with 165 of the 202 pieces being within 0.5 grain.

I contacted Lapua and they said the volume of the case is what they strive for. So I'm going to take a few of the lightest and a few of the heaviest pieces of .308 brass and fire them over the chrono. ( magnetospeed ). I'm going to do this twice so that I have twice fired brass and then measure the water weight to get an idea of the case volume.

Your 3.3gr. spread is less than +/-1% of the weight of the case with a SD of <0.55gr. I'd say that's pretty good for a mass produced case. I'm not so sure that case weight variance is a good indicator of case volume variance as each case can be a little heavier or lighter in different places of the case which may or may not affect the case's volume.

 

[ntsqd]

I think maybe my thought train isn't getting across. I'll try it differently. For the sake of discussion I'm letting the case exterior dims be exactly the same as the chamber and assuming no spring-back. I'm also assuming that the metal density does not change within one case, that it that the density is the same through-out the whole case. the first assumption is to simplify the process of thinking this thru. The second is a reasonable assumption for all metals in smaller volumes.

I see the chamber as a fixed volume. Assuming that the density of the case metal is the same from case to case then it doesn't matter where within each case that metal is distributed, it occupies the same volume if the cases weigh the same. That metal volume is subtractive from the chamber volume and the result is the total case volume.

Said a little differently, if the metal density and the case weight is the same but the distribution within the cases is different then they should still hold the same weight of water.

Unless I'm missing something.

 

[gator378]

Having a bit of statistical analysis exposure over the last 40+ years, I would bet my lunch money that the difference between cases manufactured by the same company but of different lots and sorted by tighter weight differences would be "noise", to borrow your word.

For example, two different lots of bulk XYZ brass, weight sorted by .2 or .3g instead of .5 to 1.0g would likely produce groups of cases that are as close or closer in volume. Potential exceptions being production dates that are several years apart or produced at different facilities but having the same head stamps.

Bottom line, I think one would be fine going to the LGS or Midway, or whoever and buying multiple bags/boxes of the same brand of bulk brass that may or may not be of different lots. Beyond that is overthought and maybe even a little OCD. Just my occasionally humble opinion but I'm open to being proved wrong. ;-)

I must be lucky. So far in my experience the cases are more accurate than me.

 

[six pack]

So true, Gator.....and, if the truth be told, most of us should say the same thing.