Case Volume vs. Weight

[rammac]

One of the most common disagreements we see is whether you should group your cases by volume or weight. While you'll find a lot of talk about what somebody thinks or feels, it's very rare to find a post that includes actual data. And even when data is provided it is usually not statistically relevant. Today I finally found a paper (written in 2009) that looks reputable and informative in which they actually performed a true scientific statistical analysis.

The authors tested 400 cartridge cases looking for a way to statistically identify the most important factors in optimizing cartridge cases. Their conclusions were that powder charge and bullet seating (in that order) had the greatest effect on precision (group size). But what was interesting to me was that in the process they tested the effect of other factors, including case volume and weight. While they didn't find that either had as large effect as powder charge, bullet seating depth, and primer weight, they did identify that case volume had a more significant effect than case weight.

In other words, they found that case weight was not as good a way to control group sizes, volume was better. The table below shows that volume is a more significant factor in group sizes.



A lower Pr value indicates a larger effect on group size (precision).

In reality they verify what a lot of people already knew; powder charge, seating depth, and primers have the greatest effect on group size. Another thing that their data proves is that neck inside diameter has about the same effect as case volume so from what I see after adjusting the three factors above, if you want even more precision, then you should adjust case volume and neck inside diameter (neck tension).

But I just felt that it was interesting to have finally found a study that used a truly statistical process that actually proves that case weight does not have as great as an effect on group size as case volume does. I've always tried to argue that point but without an unbiased, statistical analysis you always end the discussion by agreeing to disagree. This proves that grouping by actual case volume will have a greater effect on precision than grouping by case weights.

 

[XTR]

The real world question will be “Does it matter on the target for the type of shooting you do?” And the follow on “Is it worth your time to sort?”

 

[rammac]

The real world question will be “Does it matter on the target for the type of shooting you do?” And the follow on “Is it worth your time to sort?”

Not really, in the "real world" volume has an effect, that's what their study proved.

Your remark confuses the difference between precision and accuracy. Precision is measured (in the shooter's world) by the distance between bullet impacts while accuracy is measured by how close a bullet (or group of bullets) is to the point of aim. The study was looking at how to improve precision and regardless of how you shoot greater precision will improve your results on the target if you are accurate enough to hit your target. That's why the Marine Corps always taught Marksmanship before they taught combat shooting, Marksmanship (precision) is the basic requirement to be able to shoot accurately.

 

[Murray Brook]

I've always felt that case volume was a better road to accuracy than case weight, but is there a big enough difference between case weight to case volume to justify all the extra work in doing the case volume measure. Much less work in measuring the weight. Just my thoughts.

 

[XTR]

Not really, in the "real world" volume has an effect, that's what their study proved.

Your remark confuses the difference between precision and accuracy. Precision is measured (in the shooter's world) by the distance between bullet impacts while accuracy is measured by how close a bullet (or group of bullets) is to the point of aim. The study was looking at how to improve precision and regardless of how you shoot greater precision will improve your results on the target if you are accurate enough to hit your target. That's why the Marine Corps always taught Marksmanship before they taught combat shooting, Marksmanship (precision) is the basic requirement to be able to shoot accurately.

I completely understand the difference in precision and accuracy. My point is that for many types of shooting the precision gained is of little real value to the competitor. For example, if you are shooting on a target with a 1.5 moa X-ring then a step that requires you to sort your brass and keep it sorted into different lots and not confuse them to gain of less than 0.1 Moa in precision is not going to yield results that make a difference in your score and therefore is not worth your time.

My FTR rifles typically shoot in the 0.3s w/o sorting brass, even if sorting got me into the .2s I doubt it would make a difference at the end of the week. It’s not worth my time to sort several hundred cases for a major match.

The other side of that is that if you are in F open shooter who is shooting in a mid range match in fairly benign conditions and it’s going to be an X count race then that fraction of an MOA in precision may will make a difference in where are you finish.

 

[Ned Ludd]

F-Class matches are often won or lost by a single point or a single "X" these days, as you well know Wade. Anything that might give a shooter one less dropped "9" out the corner or one more "X" during the course of a match is worth consideration if it doesn't require excruciating amounts of time/effort. Sorting brass by weight is simple and fast, and it does make a difference, especially in the .223 Rem.

Of course, everyone has to decide for themselves whether to do it, but sorting cases by weight is really no different than pointing (or weighing) bullets, accurately weighing powder to one gazillionth of a grain, weight sorting primers, or any of the other things we do to try and gain that one extra point or "X" during the course of a match. It is not easy to ever prove beyond a shadow of a doubt that any of these efforts actually make any difference during the course of a single match. Even over a long period of time it can sometimes be difficult to demonstrate beyond any doubt that some practiced ever saved you a single point. But we do these things anyhow in the hope that they are accomplishing something, however small.

 

[rsmithsr50]

sorry but for all the time and effort spent, they then "tested " with a savage production rifle.
this would be like putting road racing tires on a elantra and using it on the indy track to establish the safest speed for any car on the track.
great tires
great track
poor test equipment

 

[AJC]

sorry but for all the time and effort spent, they then "tested " with a savage production rifle.
this would be like putting road racing tires on a elantra and using it on the indy track to establish the safest speed for any car on the track.
great tires
great track
poor test equipment

If you're looking for Trend data the only thing that matters is you use exactly the same equipment while changing the test parameters what rifle is ultimately used doesn't matter at all.

 

[jamesh]

If you're looking for Trend data the only thing that matters is you use exactly the same equipment while changing the test parameters what rifle is ultimately used doesn't matter at all.

That isn't entirely true. If you were shooting them though a blunderbuss the data would be totally worthless. Yes I understand that is an extreme example.

 

[jamesh]

I've always felt that case volume was a better road to accuracy than case weight, but is there a big enough difference between case weight to case volume to justify all the extra work in doing the case volume measure. Much less work in measuring the weight. Just my thoughts.

Some of the reason for a variance in correlation between case weight and volume is because there is some variability in the extractor groove in a given lot of cartridges which contributes to weight variation.

 

[WIB3]

These are all brand new untouched 6BR cases. They all measure 1.5550-1.5555. There weights range from 127.52(low) to 130.34(high). If the lengths are all the same, the slugs used in the drawing process should all be close in weight, the difference must be in wall thickness or head thickness. Either of will effect internal volume.

 

[rsmithsr50]

bs, how can you TEST WITH SUBSTANDARD GEAR ?
DATA COUNTS with poor test gear you get no data.
the test gear is too poor to produce reproduceable data.

If you're looking for Trend data the only thing that matters is you use exactly the same equipment while changing the test parameters what rifle is ultimately used doesn't matter at all.

 

[Ned Ludd]

Some of the reason for a variance in correlation between case weight and volume is because there is some variability in the extractor groove in a given lot of cartridges which contributes to weight variation.

FWIW - For cases with very uniform external dimensions (which cases fired in the same chamber tend to have), variance in the extractor groove and/or the depth of the primer pocket are really the only two places where case weight can change without proportionally altering the internal volume. There can certainly be variance in the extractor groove dimensions and primer pocket depth. How much is likely dependent, in part, on the specific brass manufacturer. Nonetheless, the variance between extractor grooves and or primer pocket depth between cases from the same Lot of brass generally represents a very small fraction of the total case volume.

 

[AJC]

bs, how can you TEST WITH SUBSTANDARD GEAR ?
DATA COUNTS with poor test gear you get no data.
the test gear is too poor to produce reproduceable data.

Who's to say that is Savage Barrel is substandard it is not the worst Factory Barrel out there and may just well represent the average Factory Barrel.
It may be that you see more variance in a test done in a factory Barrel than a premium match barrel and if you have a larger deviation that could give you a better indicator.

 

[rsmithsr50]

the simple FACT is that your OPINION holds no ground in the real world of science, neither does this paper.

Who's to say that is Savage Barrel is substandard it is not the worst Factory Barrel out there and may just well represent the average Factory Barrel.
It may be that you see more variance in a test done in a factory Barrel than a premium match barrel and if you have a larger deviation that could give you a better indicator.

 

[memilanuk]

After reading a little further than rsmithsr50, who probably quit reading after the first line of the abstract (OMG they used a Savage - sharpen the pitchforks and light the torches!!! ) I do have to say I have some reservations about the paper as well.

Primarily the approach - doing a full factorial with so many levels, as opposed to a screening design to whittle down the bulk of the factors that *might* matter, followed by an RSM design to quantitatively analyze those that actually *do* - which in turn resulted in trying to shoot too many groups, over too long.

When it gets to the point that there are so many targets that the horizontal spread demonstrably affects the ability of the chronograph to register 50% of the shots (much less the consistency of the remainder), combined with the significantly variation in ambient conditions from day to day, plus shooter fatigue... the test definitely could have been better designed. They did mention that they did one blocking level (presumably day 1 vs. day 2), but not much more than that. Bravo to the tester for owning up to the various matters in section 3.3, though.

One would assume that the 'instruments used' were what was available - manually measured targets, and a scale accurate to +/- 0.1 gn... which today would be a bit on the weak side at best, particularly the latter, for weighing primers. Again, those kind of details were lacking, probably because the focus of the paper was more on completing the project and getting a grade from the instructor, rather than any expectation of 'peer review' from people in the shooting community.

I'd caution against drawing *too* many conclusions from the results of this particular work... but not because of what kind of rifle was used.

 

[Bindi2]

Regardless how the data was gathered or whether anybody agrees with the means of collection it is still relevant data from the parameters used.
Whether you choose to use or ignore is your decision any thing else is just gum bashing.

 

[ronemus]

There is a lot of emphasis on case volume, but the volume that matters is when the case is under substantial pressure, what I'll call the working volume. When the pressure is released the brass springs back from the chamber walls, else you wouldn't be able to extract it! I make the argument that case weight is a better measure of working volume than the volume measured on the bench, particularly since the spring back is not going to be uniform.

I shoot mostly NRA High Power and Long Range, so I need to prep several thousand cases each year. Measuring the volume of those cases would consume more time than I am willing to invest, but weighing is a lot faster and it has been demonstrated several times that there is a strong correlation between case weight and precision. That makes weighing cases a much more useful tool for my needs.

If I was competing in benchrest and only needed to prep 20 cases a year, I could afford the time needed to measure case volume, if I believed it was a better measure for increasing precision. Part of the skill of a competitor is selecting the right tools (which includes measurements) to achieve the goal. Having healthy discussions is a great way to get us to think about what we're doing, but we also need to consider the context - sometimes the "best" measurement is impractical.

In all of this, let's try to be civil in our discussions. Some people seem to be working at getting banned from forums by adding nothing useful to the discussion and acting rudely as well. Please think about your posts and the attitude they imply - we're almost all friends here, talking about topics that are interesting to us so rudeness only serves to get you ignored.

 

[Dusty Stevens]

I think any testing is worth the effort. Once you compile the data and consider the good feedback, all it does is make your next test better. There are some on here that only show up to poo poo everything (life of the party with lots of friends im sure). Im all for testing and learning. If you see a new guy shitting on every thread just hit the ignore button. Thatll work long enough for him to get another username after getting banned.

 

[Bryan Z.]

Good to see multivariate research being done on this topic. I went ahead and did a follow up test where I held powder charge, seating, primer weight, primer seating depth, etc. constant and then varied brass weight. In a nutshell, sought to see if brass weight was a consideration after identifying the optimal charge, seating, etc. What I found was that the lightest brass showed worse precision (statistically significantly) than average and heavier brass in the lot. Overall collectively, all of the data suggests that while powder charge, seating, etc. play a large role in precision, once those factors are optimized in your load, brass weight does become a factor in precision but only the lightest brass in the lot will have poor precision. Practically speaking, seems like it’s worth the 5-10 minutes to weigh the 100 cases in the lot and cull out the lightest pieces to gain further advantage in precision.