Effects of Case Weight Variation on Accuracy

[Laurie]

Norma still advises against compressed ball powder loads but on the basis of reduced ignition consistency and efficiency as compression sees already closely aligned kernels packed too closely together and lacking spaces between them impeding the primer flame. This may or may not happen in real life, but it resonates with what the H P White people found with IMR 'log type' powders and vibratory settling.

Thinking further on this, there is actual corroboration. I've read a few times that early production lots of .458 Win Magnum sometimes gave disastrous results - very low MVs - drastically reduced MV/MEs, squib loads and misfires (bad news given this is a dangerous game cartridge!). As with most Winchester cartridges in the Olin Corporation days ball powders produced by the sister company's in St. Marks, Florida were used which only makes this type. Because the 458 Win Mag barely has a large enough case to hold enough powder to achieve a 45-cal DG cartridge's essential MVs and MEs, the charges were very heavily compressed and a heavy case-mouth crimp used to hold it all together. Newly made or reasonably fresh ammo performed within spec, but those that were a year or two old suffered terrible problems. The factory had never seen this because all development and testing was done with newly loaded cartridges. It was found that under heavy compression the kernels fused together over time producing impenetrable lumps within the charge or even the whole lot in one fused mass and ignition and charge burn suffered. Winchester had to hurriedly start load development again and use a different powder grade to reduce charge compression or even eliminate it completely. However, the replacement couldn't achieve the originally claimed performance so the cartridge failed to live up to its promises or expectations fully, at least in its early years before propellant technology caught up.

 

[mikecr]

I'm wondering whether this much variation in 5 cases shot for group would cause variations in internal capacity enough to have any, or much, effect on accuracy.

You're conceding here that your REAL concern is capacity (not weight).
This is for you to determine for yourself, as nobody here could possibly know. It's your brass, your chamber, your load, and your sizing.
I suggest you prep & weigh your cases, fully fire form them, measure their H20 capacities, and make your local correlation.

 

[Ned Ludd]

Sorry to be a fly in your ointment the mass or volume of a powder does change or can be different for the same weight. And I will say again case weight is no indicator of case volume . This may not be true for all case manufacturers. I binned Lapua before I got that far in prep and use.

Believe what you like, that doesn't make it true. In fact, your statement is just flat out wrong. The weight correlation with case volume is simple physics. It is easily demonstrable, and contrary thinking will not change the laws of physics. If your case weight and volume measurements don't show a linear correlation, you might want to look into how to determine case water volume more accurately.

As far as the mass changing for a specified weight, I'm sorry but you appear to have little grasp of physical laws. Only if one were to move to a different location where the pull earth's gravity was different would mass and weight not be identical. Even so, the change in weight would still be uniform for a given mass. That's a scenario not even worth discussing. Powder can certainly expand and contract as temperature changes, although this is an effect at the level of a single kernel and extremely small, not worth consideration for our purposes. Far more important is the packing volume, as I stated previously. Powder kernels can be irregularly shaped and don't tend to pack well. This has nothing to do with the mass or density of a single kernel, it is a byproduct of the shape and size of the kernels. Non-uniform packing of powder within the case can have a profound effect on burn rate, pressure and velocity, regardless of whether the internal case volume is the same. Again, these are simple physical parameters, I'm not sure why you are having such difficulty grasping the obvious.

 

[South Pender]

This is probably a trivial question, but, for those of you who do weight sort your cases, how do you indicate on the case which weight class it belongs to. Obviously, one would want to do this weighing and sorting only once (not each time you reload), and, therefore, there must be a way of indicating light cases, medium-weight cases, heavy cases, etc. On the 222 Rem. cases I'm working with, there's just no room on the head of the case for a scratch or something that would index the weight class. Would a tiny drop of nail polish in the extractor groove hold up on firing?

 

[South Pender]

Believe what you like, that doesn't make it true. In fact, your statement is just flat out wrong. The weight correlation with case volume is simple physics. It is easily demonstrable, and contrary thinking will not change the laws of physics. If your case weight and volume measurements don't show a linear correlation, you might want to look into how to determine case water volume more accurately.

As far as the mass changing for a specified weight, I'm sorry but you appear to have little grasp of physical laws. Only if one were to move to a different location where the pull earth's gravity was different would mass and weight not be identical. Even so, the change in weight would still be uniform for a given mass. That's a scenario not even worth discussing. Powder can certainly expand and contract as temperature changes, although this is an effect at the level of a single kernel and extremely small, not worth consideration for our purposes. Far more important is the packing volume, as I stated previously. Powder kernels can be irregularly shaped and don't tend to pack well. This has nothing to do with the mass or density of a single kernel, it is a byproduct of the shape and size of the kernels. Non-uniform packing of powder within the case can have a profound effect on burn rate, pressure and velocity, regardless of whether the internal case volume is the same. Again, these are simple physical parameters, I'm not sure why you are having such difficulty grasping the obvious.

Ned, I'm guessing that the Pearson correlation coefficient between case weight and case volume would be something like ̶ .90, maybe closer to ̶ 1.0. You mentioned some scatter about the least-squares regression line, so evidently it isn't quite ̶ 1.0. Have you seen an actual Pearson rxy value for this relationship?

 

[243winxb]

for those of you who do weight sort your cases,

Out of 100 new brass, each lot gets there own ammo box and lot number. If only a few, a plastic bag. The very heavy or lightest brass can be used to set up the neck turner or used for sighters/fouling shots.

Sorted into .3 gr groups. The main lot may have over 50 pcs in it.

Write on an adhesive label, the weight range and a lot number. Stick on box or bag.

 

[smoooth]

This is probably a trivial question, but, for those of you who do weight sort your cases, how do you indicate on the case which weight class it belongs to. Obviously, one would want to do this weighing and sorting only once (not each time you reload), and, therefore, there must be a way of indicating light cases, medium-weight cases, heavy cases, etc. On the 222 Rem. cases I'm working with, there's just no room on the head of the case for a scratch or something that would index the weight class. Would a tiny drop of nail polish in the extractor groove hold up on firing?

A Sharpie Marker swipe of your desired color across the base of the case works quite well. There are at least 6 different colors available, probably more.

Best of all, it’s easy to apply or reapply as needed and won’t gunk up your bolt or action like nail polish could.

Also, be sure to record your color coding and the corresponding case weights / load data. Put batches in bags or boxes with a note containing all the load information and color coded case weight.

 

[dstoenner]

"Cartridge brass density is 8.53 gm/cc, according to Matweb. That means 0.853 grains, in identically trimmed and primer pocket uniformed cases, will represent 0.1 grains difference in powder space. Since pressure goes up exponentially with increase in powder charge, this is about like a 0.05 grain charge difference. Not usually a significant error. If you charge cases ±0.1 grains, then figure cases ±1.7 grains are roughly equivalent to that."

Sorry the engineer is coming out in me. I read that statement and it didn't ring true. I feel it has a lot of mis-information.

So here goes my derivation of the same thing.

Brass is somewhere around 8.53 gm/cc. A gram is 15.4 grains. Therefore brass weighs ~131 gns/cc. Taking 1/131 to convert cc/gn we get .0076. We will need this later.

I personally sort my military 223 cases by 1 gn and 308/30-06 military cases by 2 gns. With Lapua I keep as a single set and only weigh the first time to get any outliers out of the mix.

I had to do some searching for the volume of lets say Varget powder per CC. Turns out that is the way the Lee poswder dispenser works and how you set it. From the tables Varget has a VMD of .0731 Meaning that 1 CC of Varget weighs 13.6 grains.

With the density of Varget you interestingly get 1 gn of case weight is equivalent to .103 gns of Varget. (.0076 x 13.6 = .103) Not the +/-.1 (.2) gn powder to 1.7 grain of case weight in the quoted statement. Instead of 1:8.5, it is more like 1:10. SO around about we came to similar numbers. 1:10 is even less critical.

But the case increase or loss by 1 gn is really a change in volume (.4% (.0076/1.87cc) per gn for 223) which would be an increase or decrease in internal pressure if we are holding our powder charge constant to within .1 gn for example. You would then need something like QuickLoad to really even estimate what is going on.

For me, this showed me that I have been sort of anal about sorting by case weight. For the most part it is not really that necessary. Even Lapua doesn't vary more than 2 gns for larger 308 or 30-06 including the outliers that I found. 6BR and 6.5x47 is even tighter and sort of proportional to raw case weight. I do believe that it can help tighten SDs and ESs for the 1000 yard line like a lot of people have already said. Since I might have all the brass I am every going to need I guess I have done my life's work for little good except to know I am coming to the line with my best foot forward.

HTH give a little perspective

David

 

[Ned Ludd]

Ned, I'm guessing that the Pearson correlation coefficient between case weight and case volume would be something like ̶ .90, maybe closer to ̶ 1.0. You mentioned some scatter about the least-squares regression line, so evidently it isn't quite ̶ 1.0. Have you seen an actual Pearson rxy value for this relationship?

South Pender - I recently posted a detailed explanation why case weight correlates with case volume in a previous thread, together with supporting data that might be of interest to you: http://forum.accurateshooter.com/threads/acceptable-brass-weight-and-volume.3971293/ (see post #19). The correlation coefficients typically range from approximately 0.75 to 0.90, indicating that there is a strong linear correlation between the x and y values, although it is not perfect (i.e. value = 1.0). Note: the graphing program I use doesn't add the +/- sign in front of the correlation coefficient, although the best line of case weight versus case volume data indeed has a negative slope.

Ideally, it would be best to plot several hundred values for case weight versus case volume. Unfortunately, it is quite painful and slow to accurately determine water volume accurately for that many cases, which is the whole point of using case weight as a surrogate measurement for case volume as it is much easier and less time consuming to sort cases by weight. Nonetheless, I have been plotting case weight versus case volume for smaller numbers of cases for years, and in my hands there is a ways a good linear correlation between the two. I have seen others post graphs of case weight versus case volume with several hundred data points and it was absolutely clear to the naked eye that there was, in fact, a linear correlation between the two. I think it is safe to conclude that when many people measure case volume and claim it does NOT correlate with case weight, it is far more likely that their case volume measurements are suspect, rather than the laws of physics suddenly stopped working. The variance between regions of the case that have the largest potential to cause a change in case weight without affecting case volume (i.e. extractor groove and primer pocket) is simply too small to have a large effect. Bubbles inside the case are probably a much bigger culprit in terms of case volume measurement error, followed by non-uniform filling of the cases (i.e. the meniscus isn't perfectly flat every time). Some time ago, I also posted a thread about the method I use to determine case water volume: http://forum.accurateshooter.com/threads/case-volume-determination-pic-heavy.3896148/. This method is both accurate and reproducible.

People are certainly free to look at the evidence themselves and believe whatever they want about whether case weight correlates with case volume; I already know the correct answer. However, the bigger question in terms of precision reloading is whether minor variance in case volume is sufficient to cause a significant and reproducible effect on pressure/velocity and group size. That is a fair question and ultimately can only be determined by the individual shooter. For someone interested in doing some simple testing for themselves, I would suggest that you weigh some pieces of brass, a sufficient number to obtain about 10 pieces each that represent both the heaviest and lightest cases found in your specific Lot# of brass. Accurately record the weight, then determine the water volume for each case. Once you have done that, load the cases identically, then shoot groups and measure velocity. By selecting only the high/low extremes of case weight, you should then have all the data you require to make an informed decision of whether sorting cases by weight is the most useful way to spend your time. Keep in mind that the absolute best outcome of sorting cases by weight is to improve the consistency of case volume within sort groups, as compared to case that have not been sorted in any way. That's all. Sorting cases by weight is not an end all/be all panacea for precision reloading.

 

[South Pender]

South Pender - I recently posted a detailed explanation why case weight correlates with case volume in a previous thread, together with supporting data that might be of interest to you: http://forum.accurateshooter.com/threads/acceptable-brass-weight-and-volume.3971293/ (see post #19). The correlation coefficients typically range from approximately 0.75 to 0.90, indicating that there is a strong linear correlation between the x and y values, although it is not perfect (i.e. value = 1.0). Note: the graphing program I use doesn't add the +/- sign in front of the correlation coefficient, although the best line of case weight versus case volume data indeed has a negative slope.

Ideally, it would be best to plot several hundred values for case weight versus case volume. Unfortunately, it is quite painful and slow to accurately determine water volume accurately for that many cases, which is the whole point of using case weight as a surrogate measurement for case volume as it is much easier and less time consuming to sort cases by weight. Nonetheless, I have been plotting case weight versus case volume for smaller numbers of cases for years, and in my hands there is a ways a good linear correlation between the two. I have seen others post graphs of case weight versus case volume with several hundred data points and it was absolutely clear to the naked eye that there was, in fact, a linear correlation between the two. I think it is safe to conclude that when many people measure case volume and claim it does NOT correlate with case weight, it is far more likely that their case volume measurements are suspect, rather than the laws of physics suddenly stopped working. The variance between regions of the case that have the largest potential to cause a change in case weight without affecting case volume (i.e. extractor groove and primer pocket) is simply too small to have a large effect. Bubbles inside the case are probably a much bigger culprit in terms of case volume measurement error, followed by non-uniform filling of the cases (i.e. the meniscus isn't perfectly flat every time). Some time ago, I also posted a thread about the method I use to determine case water volume: http://forum.accurateshooter.com/threads/case-volume-determination-pic-heavy.3896148/. This method is both accurate and reproducible.

People are certainly free to look at the evidence themselves and believe whatever they want about whether case weight correlates with case volume; I already know the correct answer. However, the bigger question in terms of precision reloading is whether minor variance in case volume is sufficient to cause a significant and reproducible effect on pressure/velocity and group size. That is a fair question and ultimately can only be determined by the individual shooter. For someone interested in doing some simple testing for themselves, I would suggest that you weigh some pieces of brass, a sufficient number to obtain about 10 pieces each that represent both the heaviest and lightest cases found in your specific Lot# of brass. Accurately record the weight, then determine the water volume for each case. Once you have done that, load the cases identically, then shoot groups and measure velocity. By selecting only the high/low extremes of case weight, you should then have all the data you require to make an informed decision of whether sorting cases by weight is the most useful way to spend your time. Keep in mind that the absolute best outcome of sorting cases by weight is to improve the consistency of case volume within sort groups, as compared to case that have not been sorted in any way. That's all. Sorting cases by weight is not an end all/be all panacea for precision reloading.

Thanks, Ned. That's really terrific information. The .75 to .90 correlation makes a lot of sense. Actually, I thought it might be even higher.

 

[ronemus]

I did an experiment several years ago to determine just how much effect brass weight has on .223 loads. I used WW brass (sized, trimmed and deburred, primer pockets uniformed, flash holes deburred, and neck turned) , WSR primers, charges of RL-15 or N-550 powder weighed to 0.1 gr, and 75 gr A-Max bullets. Using the lightest and heaviest cases (sorted from 1000 once-fired I had on hand), I had two lots of 10 cases with a 3 gr difference in weight. The average muzzle velocity difference was 16 fps, just a bit more than the 12 fps due to 0.1 gr of powder. I choose to sort 0.5 gr lots of .223 brass for my long range loads, but the effect will only matter at 800-1000 yards - the vertical displacement on the target from such a small velocity change is negligible at shorter distances. Unless you control all other sources of variation, the effect of brass weight is negligible. I also shoot .284, and because the brass is twice as heavy I batch in 1 gr lots.

 

[Bindi2]

Believe what you like, that doesn't make it true. In fact, your statement is just flat out wrong. The weight correlation with case volume is simple physics. It is easily demonstrable, and contrary thinking will not change the laws of physics. If your case weight and volume measurements don't show a linear correlation, you might want to look into how to determine case water volume more accurately.

As far as the mass changing for a specified weight, I'm sorry but you appear to have little grasp of physical laws. Only if one were to move to a different location where the pull earth's gravity was different would mass and weight not be identical. Even so, the change in weight would still be uniform for a given mass. That's a scenario not even worth discussing. Powder can certainly expand and contract as temperature changes, although this is an effect at the level of a single kernel and extremely small, not worth consideration for our purposes. Far more important is the packing volume, as I stated previously. Powder kernels can be irregularly shaped and don't tend to pack well. This has nothing to do with the mass or density of a single kernel, it is a byproduct of the shape and size of the kernels. Non-uniform packing of powder within the case can have a profound effect on burn rate, pressure and velocity, regardless of whether the internal case volume is the same. Again, these are simple physical parameters, I'm not sure why you are having such difficulty grasping the obvious.

You can believe what you like and quote what you like. I have been at the pointy end of this game for 50 years starting when there was little to no information available. Simple physics and maths can be proved wrong. I have just had an issue with a young fella who developed an attitude with me. He would not believe he was wrong he couldn't be wrong , the computer is never wrong. I pointed out his input data was wrong. CHECK YOUR DATA.

 

[Ned Ludd]

You can believe what you like and quote what you like. I have been at the pointy end of this game for 50 years starting when there was little to no information available. Simple physics and maths can be proved wrong. I have just had an issue with a young fella who developed an attitude with me. He would not believe he was wrong he couldn't be wrong , the computer is never wrong. I pointed out his input data was wrong. CHECK YOUR DATA.

Again, believe as you wish. Years of experience mean very little if you're refuse to believe evidence that's staring you in the face. I do know something about it, and I am perfectly satisfied with the quality of the data I have produced and used. You're attempting to claim that a linear relationship between case weight and case volume does NOT exist. What data or evidence do you have to support your claim? As far as I can tell, it's solely your opinion. I'm sorry, but opinions don't count here. Neither do graphs with random scatter plots of case weight versus case volume. Poor water measurement technique is a far better explanation for an apparent non-linear scatter plot than that the laws of physics suddenly changed. So I'll ask again, what actual evidence do you have to support your claim that there is not a linear relationship between case weight and case volume? If you believe that conclusion as you have stated, surely you have some reason for believing it. Here's a perfect opportunity to support your claim with evidence.

 

[Bindi2]

Again, believe as you wish. Years of experience mean very little if you're refuse to believe evidence that's staring you in the face. I do know something about it, and I am perfectly satisfied with the quality of the data I have produced and used. You're attempting to claim that a linear relationship between case weight and case volume does NOT exist. What data or evidence do you have to support your claim? As far as I can tell, it's solely your opinion. I'm sorry, but opinions don't count here. Neither do graphs with random scatter plots of case weight versus case volume. Poor water measurement technique is a far better explanation for an apparent non-linear scatter plot than that the laws of physics suddenly changed. So I'll ask again, what actual evidence do you have to support your claim that there is not a linear relationship between case weight and case volume? If you believe that conclusion as you have stated, surely you have some reason for believing it. Here's a perfect opportunity to support your claim with evidence.

Your evidence is suspect. Your attitude is coarse. I believe what I see after I ask why is it so and check it more than once and ongoing because things occur. Do you measure your cases before fireforming or after how many firings, before FLS or after just for starters. Theory is only a starting point for practical work for real time answers.

 

[boltfluter]

Bindi2,

Nedds attitude is not course, it is based on backed up facts and testing. If anyone's attitude is course it is yours. Many people think because they are older they are always correct. I have seen many people do things wrong for 50 yrs or more! Many people don't like to be challenged about their beliefs. If you have the info to back up the claim, why would you not! This forum is truelly based on getting to the right conclusion, which is backed up with facts or testing. Your beliefs, without facts or proof are just that, your beliefs. We are all ears! Some people just do not like to be challenged on their beliefs. Nothing wrong with that. We are here to learn, nothing more. As they say, if the kitchen is too hot, then exit the building.

Paul

 

[Bindi2]

Bindi2,

Nedds attitude is not course, it is based on backed up facts and testing. If anyone's attitude is course it is yours. Many people think because they are older they are always correct. I have seen many people do things wrong for 50 yrs or more! Many people don't like to be challenged about their beliefs. If you have the info to back up the claim, why would you not! This forum is truelly based on getting to the right conclusion, which is backed up with facts or testing. Your beliefs, without facts or proof are just that, your beliefs. We are all ears! Some people just do not like to be challenged on their beliefs. Nothing wrong with that. We are here to learn, nothing more. As they say, if the kitchen is too hot, then exit the building.

Paul

Sorry Paul Neds attitude is coarse personal attack. The whole point I am making is check your own data many times don't believe what you read on the Internet it could be very wrong and dangerous. Many have learnt this the hard way. I have proved the data in question to my satisfaction contrary to Ned. I don't publish data as I wont be responsible for others not understanding what I said. When the furnace is roaring you are at the pointy end . Long term learning trumps upstarts till they have time up. In the mean time I will continue shooting and learning knowing that things are not what they seem . Question with your knowledge being so good why are the Aussie shooters leading in so many rifle events. Why is it so.

 

[Ned Ludd]

I have proven the answer, and provided rock solid data to support my claim. You chose not to support your claim at all, which effectively means your claim HAS no support, other than it being your personal opinion. You can certainly choose to ignore the laws of physics and actual data whenever it disagrees with your opinion. However, I think anyone reading this post is fully capable of weighing what they have read and deciding for themselves. The first dead giveaway is that when someone resorts to name-calling, it's because they have no other argument.

Good day to you, sir.

 

[Bindi2]

The first dead giveaway is that when someone resorts to name-calling, it's because they have no other argument.

Good day to you, sir.

Your quote seems to fit you very well

 

[davidjoe]

If you think it would help to cull or sort by weight group, I say do it. Confidence in one's performance is, in my opinion, a bigger factor in getting the results you want than most of the little tuning tricks one chooses to employ.

Personally I have sorted by weight groups but I have not noticed any difference, probably because in my case, the nut behind the bolt is still the biggest factor.

Yep. The barrel’s sporadic shot to shot erosion itself, and the random distribution, bunching and cyclical removal of fouling left behind, probably has a more significant effect on pressure variance at the muzzle than small brass weight and shape differences. All of them being very small.

A potential litmus test is to shoot a group in a tunnel with the exact same piece of brass, reloaded. Then compare the group to one from alike brass. Then a group from the extreme spread. Then repeat all that several times and look for a pattern. I’d be surprised is any pattern other than a gradually wearing out barrel can be discerned.

 

[dstoenner]

I have a question for the group. Has anybody taken 2 extreme weight cases from a single date code, single vendor and milled them in half so you could see the wall thickness as well as the case head dimensions and see where the weight changes are occurring from?

If that has been done what has it shown?

David