Measuring case internal volume

[Tiratore]

If case weight falls within a fairly narrow and consistent range, adding the extra weight of water plus the case shouldn't change the slope of the line, only the y-intercept. Case volume will always show the general trend of decreasing as case weight increases. Your first method of plotting the data is misleading with regard to the effect of case weight on case capacity. It's merely a way of making the data look much better than it really is because the critical value, which is the actual water weight or case volume, becomes proportionally reduced when the weight of the case is also included. For example, take the highest (~90 gr) and lowest (~84.7 gr) case weight values. When graphed appropriately to illustrate the inverse relationship between case weight and case volume as shown directly above, these values clearly show up as outliers. In your original graphing approach, they would be spot on a trend line, if one were present, which is misleading because the trend line that is implied by the original scatter plot you showed would have a positive slope. Perhaps it's only my impression and maybe you didn't mean to imply that, but the original graph certainly gave me that impression. Case volume will always show a general decrease (negative slope) as case weight increases, regardless of whether the weight of the case is included or not.

You are correct. On every count. The original graph can be misleading.

 

[dmoran]

I have a tool I made an I get the same result 10 times all within .01 Gr . It is not total case capacity because is done off the shoulder and a portion of the neck .
The cases must be fired in the gun and un touched when checking . Don't have a problem keeping ES of 5 and under using a charge master . Larry

Larry -
What scale are you using to cue your volumes to: "all within .01 Gr"?
And again, if it is your ChargeMaster, they are capable of +/- 0.1 (2-tenths, not 1-hundredth)
Also, what chronograph are you using?... and what is its accuracy capability?

 

[savagedasher]

You are correct. On every count. The original graph can be misleading.

Total weight does not mean anything about the volume . I have small dasher cases with the same internal volume that weighs over 1 full gr different .
Any case that has more then .02 differences changes ES . With cases that the internal in the .01 range I get ES less then 5 and SD of 3 on 10 shots that is the within margin of error in the Crony . Larry

 

[SheepDog]

Weighing dry cases is "representative" of the volume only if the case external dimensions are the same.
Filling the case with water for volume weight can vary if you are measuring to the shoulder and not to the mouth of the case.
If the shoulder and mouth of the case are of different thicknesses and the body of the cases are lighter you can have more volume to the shoulder with the same weight cases but if you fill to the mouth it will have the same volume with the same weight cases even though there is more or less usable volume for the actual powder charge. I have always segregated my cases by weight then after firing them by volume filling to the powder level in the case. I stopped checking the volume a long time ago because brass is about seven times as dense as water so a 7 grain difference is only 1 grain difference in water capacity. Powder densities vary between the same as water and slightly denser than water. If you are segregating your cartridges by 2 grain increments then the variation in powder space is only 0.28 grain. The affect that has on hunting ammunition is negligible.
For long range target shooting It would have more effect if all the case volume was used and the pressures were near or exceeded maximum suggested loads. This istrue any time you are working on the ragged edge in temperature extremes and longer ranges. I have seen no value in the extra work in my shooting which is limited to about 200 yards.

 

[Ned Ludd]

You are correct. On every count. The original graph can be misleading.

Actually, I made an error in that the density of brass is ~8.5 times greater than that of water. Therefore, a heavier case filled with water will weigh more than a lighter case even though it has less internal water volume on average. The heavier case gains more weight from the density of the extra brass than it loses from the decrease in water volume. So the slope of the trend line comparing filled cases to empty should actually have a positive slope as shown in your graph, not negative as I stated above. My only reason for pointing any of this out to any readers of this thread is simply that although the trend of heavier cases having less internal volume will generally hold true, there will always be a certain number of "outliers" that fall some distance from the trend line. Even for those that do these types of sorting on a regular basis, it still can be very confusing at times. As I mentioned, I do sort cases by weight, simply because it is easy and relatively fast and I accept the fact that there will be some outliers. I think overall, weight sorted cases will still have more uniform internal volume than unsorted, even with the outliers.

 

[dmoran]

I have a tool I made an I get the same result 10 times all within .01 Gr . It is not total case capacity because is done off the shoulder and a portion of the neck .
The cases must be fired in the gun and un touched when checking . Don't have a problem keeping ES of 5 and under using a charge master . Larry

Larry -
What scale are you using to cue your volumes to: "all within .01 Gr"?
And again, if it is your ChargeMaster, they are capable of +/- 0.1 (2-tenths, not 1-hundredth)
Also, what chronograph are you using?... and what is its accuracy capability?

​

 

[Tiratore]

Ignoring for the moment the argument that case weight as determined by common reloading balances is an accurate surrogate for internal case volume at the critical time during the pressure transient, here are some thoughts on the contribution of case weight.


In testing the 20 VarTarg, we have observed weights of cases from several sources vary approximately 1 grain, from the mean to the heaviest or lightest within one brand/source of cases. Sample sizes were larger than 35 cases each. Firing many rounds with known case weight over the chronograph showed a 1 grain change in case weight produces an 11 fps change in muzzle velocity, all other factors remaining constant within the precision of normal reloading equipment and component uniformity. When plotted, there is much scatter, but the slope of the trend line appears to be common to most case sources and to the entire database.


Similarly, in the region of interest, a 1 grain change in powder charge causes a 197 fps change in muzzle velocity.


Therefore, the variation in charge that would cause the same effect on muzzle velocity as unsorted cases is 0.056 grains of powder (if I have not had yet another senior moment).


Although some reloading scales read to 0.02 grains, precision does not necessarily match this sensitivity. At least per the testing I’ve conducted in my real-world basement. It appears that +/- 0.1 grains over a batch of individually trickled charges is about what can be expected.


The result of not sorting cases, at least with cases we have used, has an arguably small effect, perhaps vanishingly small, compared to the multitude of other variables involved in producing precision reloads.

 

[Hermantk]

I'll probably regret this, but ...

The importance of many reloading variables can be determined by measuring or calculating the related velocity variation. The importance of many others can’t. Case volume is probably one that can, so we can use velocity variation to assess volume’s effect on target performance.


Because of the difficulty in measuring case volume directly (surface tension, bubbles, etc.) by filling cases with a medium such as water with detergent, powder, etc., I submit without proof that weight measurement gives a better indication of volume than direct measurement. The volume of a cartridge case is equal to the chamber volume minus the volume of the metal making up the case and primer, minus the volume of the extractor groove. The density of cartridge brass is a constant, for all practical purposes. Within a brand, the volume of the extractor groove is likewise constant. The variation of volume within a cartridge case (variation, not the actual value) can be determined accurately by weighing representative cartridge cases.


I have measured the weight of 20 VarTarg cases made from several parent cases. For example, weights of cases made from Remington 221 Fireball cases have an average of 82.65 grains, and a standard deviation of 0.33 grains. I have fired cases of different weights and determined the effect of case weight upon muzzle velocity. The standard deviation in velocity corresponding to that case weight distribution is 3.6 fps. A typical standard deviation in velocity seen from ammunition made from such cases is 20 fps, due to all causes including case volume.


It is possible to calculate the contribution of case volume to the total velocity variation from the above data:


The square of the SD of a population’s velocity equals the sum of the squares of the velocity variations due to each of those independent factors that cause velocity variation. SD2 is thus equal to 400. The implied SD due to case weight variation of Remington cases is 3.6, and (3.6)2 = 12.96, and that subtracted from 400 is 387. The square root of that is 19.67. So, the standard deviation of the velocity can be reduced from 20 to 19.67 by removing any variation in case weight. It would seem the sorting by case weight within a batch of cases by one manufacturer has an immeasurably small effect. (apparently exponents don't come through correctly, but you've probably already figured SD2 is supposed to be SD squared, etc.)

I'll probably regret this, but ...

The importance of many reloading variables can be determined by measuring or calculating the related velocity variation. The importance of many others can’t. Case volume is probably one that can, so we can use velocity variation to assess volume’s effect on target performance.


Because of the difficulty in measuring case volume directly (surface tension, bubbles, etc.) by filling cases with a medium such as water with detergent, powder, etc., I submit without proof that weight measurement gives a better indication of volume than direct measurement. The volume of a cartridge case is equal to the chamber volume minus the volume of the metal making up the case and primer, minus the volume of the extractor groove. The density of cartridge brass is a constant, for all practical purposes. Within a brand, the volume of the extractor groove is likewise constant. The variation of volume within a cartridge case (variation, not the actual value) can be determined accurately by weighing representative cartridge cases.


I have measured the weight of 20 VarTarg cases made from several parent cases. For example, weights of cases made from Remington 221 Fireball cases have an average of 82.65 grains, and a standard deviation of 0.33 grains. I have fired cases of different weights and determined the effect of case weight upon muzzle velocity. The standard deviation in velocity corresponding to that case weight distribution is 3.6 fps. A typical standard deviation in velocity seen from ammunition made from such cases is 20 fps, due to all causes including case volume.


It is possible to calculate the contribution of case volume to the total velocity variation from the above data:


The square of the SD of a population’s velocity equals the sum of the squares of the velocity variations due to each of those independent factors that cause velocity variation. SD2 is thus equal to 400. The implied SD due to case weight variation of Remington cases is 3.6, and (3.6)2 = 12.96, and that subtracted from 400 is 387. The square root of that is 19.67. So, the standard deviation of the velocity can be reduced from 20 to 19.67 by removing any variation in case weight. It would seem the sorting by case weight within a batch of cases by one manufacturer has an immeasurably small effect. (apparently exponents don't come through correctly, but you've probably already figured SD2 is supposed to be SD squared, etc.)

I'll probably regret this, but ...

The importance of many reloading variables can be determined by measuring or calculating the related velocity variation. The importance of many others can’t. Case volume is probably one that can, so we can use velocity variation to assess volume’s effect on target performance.


Because of the difficulty in measuring case volume directly (surface tension, bubbles, etc.) by filling cases with a medium such as water with detergent, powder, etc., I submit without proof that weight measurement gives a better indication of volume than direct measurement. The volume of a cartridge case is equal to the chamber volume minus the volume of the metal making up the case and primer, minus the volume of the extractor groove. The density of cartridge brass is a constant, for all practical purposes. Within a brand, the volume of the extractor groove is likewise constant. The variation of volume within a cartridge case (variation, not the actual value) can be determined accurately by weighing representative cartridge cases.


I have measured the weight of 20 VarTarg cases made from several parent cases. For example, weights of cases made from Remington 221 Fireball cases have an average of 82.65 grains, and a standard deviation of 0.33 grains. I have fired cases of different weights and determined the effect of case weight upon muzzle velocity. The standard deviation in velocity corresponding to that case weight distribution is 3.6 fps. A typical standard deviation in velocity seen from ammunition made from such cases is 20 fps, due to all causes including case volume.


It is possible to calculate the contribution of case volume to the total velocity variation from the above data:


The square of the SD of a population’s velocity equals the sum of the squares of the velocity variations due to each of those independent factors that cause velocity variation. SD2 is thus equal to 400. The implied SD due to case weight variation of Remington cases is 3.6, and (3.6)2 = 12.96, and that subtracted from 400 is 387. The square root of that is 19.67. So, the standard deviation of the velocity can be reduced from 20 to 19.67 by removing any variation in case weight. It would seem the sorting by case weight within a batch of cases by one manufacturer has an immeasurably small effect. (apparently exponents don't come through correctly, but you've probably already figured SD2 is supposed to be SD squared, etc.)

According to tech support at Lapua USA (Capstone), the difference in cartridge weight is due to the differences in amount of material removed in cutting the extractor groove. arrrgggghhhhhhh

 

[Hermantk]

According to tech support at Lapua USA (Capstone), the difference in cartridge weight is due to the differences in amount of material removed in cutting the extractor groove. arrrgggghhhhhhh

It just occurred to me that, once the weight or volume sorting has been done...if i do not shoulder bump every case to exactly the same base-to-shoulder length, then i will have artificially introduced differences in internal volume. So i spend hours upon hours of volume sorting and then, after firing, bump one case to 1.805" and another to 1.803" and i have different case volumes. Or am i missing something here?

 

[carnaby]

What is an efficient way to measure the internal case volume?

There is now an efficient way:

Dr. Triplett’s Cartridge Case Volumizer – Bison Ops

Uses air and your reloading press along with a computer. Takes around 3 seconds per case to measure the volume.

 

[mikecr]

It just occurred to me that, once the weight or volume sorting has been done...if i do not shoulder bump every case to exactly the same base-to-shoulder length, then i will have artificially introduced differences in internal volume. So i spend hours upon hours of volume sorting and then, after firing, bump one case to 1.805" and another to 1.803" and i have different case volumes. Or am i missing something here?

You're right, and the less you size cases the better measured capacities will match.
But even more important to the whole endeavor is chamber clearance to the brass character itself. That is, the individual case stress-strain potentials.
This leaves us with relative capacities and more to the matching chore than most consider.

Given this, it seems little sense to me that FL sizing folks would bother with capacity measurements, and trying to match them. They're changing brass character with each cycle, and even trimming an attribute of capacity. Brushing matched brass off the bench into a trash can? Really? Gonna go back & re-match all that?

Overall, capacity matching and gains are there to get, but not with shortcuts.

 

[M-61]

Post really covered a few years