Effect of Case Capacity on Velocity ES/SD

[Auto-X Fil]

Hello,

I am able to compute the effects of most of the variables that go into producing accurate loads, but I'm coming up short on hard data about case volume. There are innumerable threads with uncorroborated opinions about the importance of case volume (and flash hole prep, and neck turning, etc.), but I can't find a single instance of someone isolating the case volume variable and producing empirical data.

Obviously the best thing for any particular load is to do is sort my cases, charge with the best precision I can muster, and chronograph them. However, I shoot a few calibers, and develop loads for a bunch of friends, so that's not an experiment I want to repeat over and over until I've picked out how the results correspond to bullet weight, powder chosen, and nominal capacity. If there's work by German Salazar or Bryan Litz or someone similarly dedicated that provides a more general guideline for case capacity vs velocity variation, I'd love to read it.

Does anyone know of any such work?

Thanks!
-Phil

 

[fredo]

To over-simplify: velocity is generated by case pressure.

Case pressure is a factor a powder charge being ignited in a certain volume.

If charge weight and/or volume vary, the pressure generated will vary, as well.

The same charge weight ignited in a smaller volume will generate more pressure.
Conversely, the same charge in a larger volume will generate less pressure.

Any pressure variance will manifest itself in velocity spread.

Regarding powder burn rate:

An equal volume of faster burning powder will create more pressure than a slower burn powder, if ignited in a constant volume. This is why you see certain powder require less charge weight in a certain cartridge to achieve desired speed. There are examples of this everywhere, they're called reloading manuals


This simplistic explanation is assuming primer spark & mechanical factors (like neck tension, bullet weight/shape) are constants and will not alter velocity spread in any way.

A program like Quickload is a great tool to help understand & comprehend the symmetry between case capacity, bullet choice, powder burn rate, case pressure, barrel length & net velocity. Definitely worth the investment, if you're a knowledge seeker...

 

[amlevin]

To over-simplify: velocity is generated by case pressure.

Case pressure is a factor a powder charge being ignited in a certain volume.

If charge weight and/or volume vary, the pressure generated will vary, as well.

The same charge weight ignited in a smaller volume will generate more pressure.
Conversely, the same charge in a larger volume will generate less pressure.

Any pressure variance will manifest itself in velocity spread.

Regarding powder burn rate:

An equal volume of faster burning powder will create more pressure than a slower burn powder, if ignited in a constant volume. This is why you see certain powder require less charge weight in a certain cartridge to achieve desired speed. There are examples of this everywhere, they're called reloading manuals


This simplistic explanation is assuming primer spark & mechanical factors (like neck tension, bullet weight/shape) are constants and will not alter velocity spread in any way.

A program like Quickload is a great tool to help understand & comprehend the symmetry between case capacity, bullet choice, powder burn rate, case pressure, barrel length & net velocity. Definitely worth the investment, if you're a knowledge seeker...

As they say, "This all looks good on paper" but has anyone actually taken cases that are alike in all parameters other than volume and tested the theory?

 

[Auto-X Fil]

I know QuickLoad has strengths and weaknesses, but it makes sense that pressure vs. capacity would be something that can be calculated with good accuracy. Perhaps it's time to pony up. Honestly, the biggest reason I haven't purchased it is because it's a Windows-only program that comes on a CD-ROM...

As they say, "This all looks good on paper" but has anyone actually taken cases that are alike in all parameters other than volume and tested the theory?

Exactly what I'm looking for... ideally with a variety of cartridges and powders and bullets, although I understand how additional variables dramatically increase experiment complexity. It just seems like the sort of experiment that might exist in one of Bryan's books, so I don't want to go do some digital or real-world effort at great time and expense, if I can buy a $40 book and get much more general data. I'm just not sure if this data exists, or where.

 

[Sheldon N]

Some discussion about it in this thread... with some (inconclusive) empirical test results.

http://forum.accurateshooter.com/threads/sorting-cases-by-weight.3898477/

Something to consider.... Selection of the best shooting powder charge weight node, using the components that exhibit the best synergy with that node in your gun, and maintaining consistency in brass prep (neck turning, trimming, annealing, etc) all have a much bigger effect than the potential effect of trying to sort cases by weight or volume.

Secondly, the way that the math of stacked variability works means that any potential improvement in velocity consistency from sorting cases by volume won't really appear until you have a very low SD to start with. In other words, if your baseline SD is 12 and sorting cases by volume could result in a SD reduction of 2 fps by itself, that 2 fps "improvement" is almost totally lost in the noise of the 12 fps baseline SD.

 

[savagedasher]

To over-simplify: velocity is generated by case pressure.

Case pressure is a factor a powder charge being ignited in a certain volume.

If charge weight and/or volume vary, the pressure generated will vary, as well.

The same charge weight ignited in a smaller volume will generate more pressure.
Conversely, the same charge in a larger volume will generate less pressure.

Any pressure variance will manifest itself in velocity spread.

Regarding powder burn rate:

An equal volume of faster burning powder will create more pressure than a slower burn powder, if ignited in a constant volume. This is why you see certain powder require less charge weight in a certain cartridge to achieve desired speed. There are examples of this everywhere, they're called reloading manuals


This simplistic explanation is assuming primer spark & mechanical factors (like neck tension, bullet weight/shape) are constants and will not alter velocity spread in any way.

A program like Quickload is a great tool to help understand & comprehend the symmetry between case capacity, bullet choice, powder burn rate, case pressure, barrel length & net velocity. Definitely worth the investment, if you're a knowledge seeker...

First time I have seen a direct answer to a question with out a song and dance. Larry

 

[fredo]

As they say, "This all looks good on paper" but has anyone actually taken cases that are alike in all parameters other than volume and tested the theory?

Yes.
I like refer to it as "load development".
Would suggest shooters to try it sometime, and quit getting their load data from the friggin' internet.

Amazing what one can find on their own, if they'd only listen to what their rifle, the chrono, and the target are telling them...

Every new rifle & every new powder/bullet combo tested constitutes an "experiment". Why would anyone rationally assume that somebody else could tell them what their rifle likes, over what one could find out on their own?

Isn't that part of the fun of reloading? Or, has the task devolved into wanting a magical "turn key" recipe book that'll negate the work involved to develop a good load?

 

[Auto-X Fil]

Sheldon-

Thank you. That's fantastic, especially the scatter-plots.

Although I'm still curious about the ability to quantify it, the fact that you and a few others in that thread tried and were unable to sufficiently isolate the capacity variable enough to draw a strong correlation tells me:

1) Given my skill level, running that experiment would be futile for me.
2) I've been able to run the numbers and show that an SD of 10fps is close enough for most of the shooting I do... so as long as I meet that with what I consider acceptable reloading effort, I don't need to concern myself with which variables are the low-hanging fruit.

 

[22BRGUY]

And then there's change in case volume created by changes in seating depth.

 

[savagedasher]

And then there's change in case volume created by changes in seating depth.

Case volume being used . Correct
Larry

 

[ireload2]

You might take a look at the gas laws where P x V = P x V and PV = nRT

PV = PV means there is a direct relationship between pressure and volume. The only problem is you have no way to measure your pressures except indirectly with a chronograph.

Ideal gas law
Pv = nRT here
https://www.khanacademy.org/science...ry/ideal-gas-laws/v/ideal-gas-equation-pv-nrt

 

[Laurie]

It's an accepted fact that different makes of brass in a cartridge which have different capacities give different performance to the point where knowledgeable handloaders use varying charge weights of the same powder with a single bullet model. A very common example is 308 Win where high-capacity Winchester brass allows (in fact requires) heavier loads than Lapua. In my minimum-SAAMI chambered 308 FTR rifles, 1980s era fireformed 168gn Norma cases from a super-thin period in Norma production hold 57.4gn water, Winchester 57.0gn and Lapua 56.1gn. The Norma brass internal capacity is 2.5% higher than that of Lapua, and that does make a difference to both loads and pressures. Other direct comparisons such as ES/SD are difficult if not meaningless because other factors also vary such as the brass alloy ingredients mix percentages, differences in how they behave under pressure, differing neck tensions.

There is a general view these days that with high quality brass from a single production lot, that there is much less benefit in weight batching than used to be believed. Many very successful long-range precision shooters don't bother with this, or if they do weigh cases just look for the odd way out at the edges of of the distribution curve to be culled out. Some people on this forum have always (probably correctly) derided the view that weight is a guide to internal capacity anyway and use volume based batching techniques such as weighing how much fine grade ball powder cases hold. (I suspect that has its own inbuilt variables too such as how do you know how consistently the powder settles unless you go yet another stage further and vibrate the filled case to get further settling!)

I may be able to run such a desired test though later this year. I bought 300 Hornady 6.5mm Creedmoor cases back in the early days of the cartridge and they have the largest weight distribution I've seen in modern commercial brass - would you believe extreme spreads of around 7gn in a 160 odd gn weight average? They do have different internal capacities too, 53.4gn water for a sample heavy example and 53.7gn for the light. It'll be quite simple to load up say 15 of each with a load combination that the barrel / cartridge likes and see what the average, and ES values are for each. This assumes of course - and assumptions are always dangerous - that Hornady used the same brass mix etc for both versions so there are no other variables in play. In any event, they were bought together, have had the same treatment, the key one being flash-hole uniforming / deburring (a very necessary step, believe me!), the same small number of resizings / firings and have been neck-annealed, as I wasn't at all happy about the factory annealing.

 

[Ned Ludd]

I know QuickLoad has strengths and weaknesses, but it makes sense that pressure vs. capacity would be something that can be calculated with good accuracy. Perhaps it's time to pony up. Honestly, the biggest reason I haven't purchased it is because it's a Windows-only program that comes on a CD-ROM...



Exactly what I'm looking for... ideally with a variety of cartridges and powders and bullets, although I understand how additional variables dramatically increase experiment complexity. It just seems like the sort of experiment that might exist in one of Bryan's books, so I don't want to go do some digital or real-world effort at great time and expense, if I can buy a $40 book and get much more general data. I'm just not sure if this data exists, or where.

Quickload will run on a Mac, no problem. All you need is some PC-emulation software. Lots of freeware available that will do it. I chose to purchase a relatively inexpensive program called "Crossover".

To your original question, if you have access to an analytical balance with readability to ~0.1 mg (or less), you can easily do a simplified test yourself. Rather than trying to test hundreds of cases that may vary only minutely in terms of case volume, or may simply vary in weight due to differences in the extractor grooves, you could sort ten cases (each) that represent both the lightest and heaviest weight range from that particular lot (i.e the "extremes"). Load them up identically; same bullet, same charge weight, same seating depth, etc., and determine velocities. Afterward, determine case volume for the fired cases, it's pretty easy:
http://forum.accurateshooter.com/threads/case-volume-determination-pic-heavy.3896148/

If the average velocities determined for the light set and the heavy set aren't different by more than the SD for the two sets, then they're not statistically different and you'll never be able to shoot the difference. Further, you will not be able to use case weight as a reliable indicator of relative case volume.

If the average velocities for the light and heavy case sets do differ and are statistically significant, you can decide whether to go the extra mile and measure for a whole bunch of cases across the entire weight spectrum.

The bottom line is that case volume and pressure are dependent. Change one and you'll change the other. However, from a practical viewpoint, if there isn't a convenient method to measure case volume that is less time-consuming than hand weighing individual cases with water, very few will routinely do it, and the benefits are dubious.

Rather than spend any time doing even such a limited test as I outlined above, I have simply assumed that there is a general [inverse] correlation between case volume and case weight, and that it is only minimally impacted by other variables such as differences in the extractor grooves or other processing steps done during reloading. I can sort enough cases for a match by weight in only a few minutes using the same analytical balance I use to weigh powder. It's easy and doesn't take much time. I figure that even if there are other factors involved in case weight variance that don't affect internal volume, weight sorting cases isn't likely to make the volume variance increase appreciably, and there may be some small benefit. Knowing this, I don't mind the small amount of time it takes to do it. I would not spend the time determining case volume by the water weight method for the hundreds of cases I routinely fire from different rifles. YMMV.

 

[savagedasher]

I sort all my competition brass by internal volume . Is it worth while ? Yes it is for me. To me why would you weight the powder to a single kernel and put the powder case that has 50 kernel difference .
I keep my ES under 5 . You can decide I know what works for me. Larry

 

[Hengehold]

Hello,

I am able to compute the effects of most of the variables that go into producing accurate loads, but I'm coming up short on hard data about case volume. There are innumerable threads with uncorroborated opinions about the importance of case volume (and flash hole prep, and neck turning, etc.), but I can't find a single instance of someone isolating the case volume variable and producing empirical data.

Obviously the best thing for any particular load is to do is sort my cases, charge with the best precision I can muster, and chronograph them. However, I shoot a few calibers, and develop loads for a bunch of friends, so that's not an experiment I want to repeat over and over until I've picked out how the results correspond to bullet weight, powder chosen, and nominal capacity. If there's work by German Salazar or Bryan Litz or someone similarly dedicated that provides a more general guideline for case capacity vs velocity variation, I'd love to read it.

Does anyone know of any such work?

Thanks!
-Phil

I have conducted tests by sorting brass by weight. I shot groups with the flow extreme and high extreme weights of the Lapua brass that I had. At 600 yards I did not see a point of impact change between the high extreme and low extreme breast weights . My application was as a NRA prone shooter and a point of impact change of 1/4 min or greater would have been obvious pain on paper .


-Trevor

 

[Keith Glasscock]

Let me preface my comment with a simple truth:

We as competitors and shooters really don't care about ES or SD. What we really want is point of impact consistency (precision).
Because we have difficulty doing other experiments, we tend to use velocity consistency as a surrogate for what we really want.

Therein lies the rub!

While we have a reasonable understanding of external ballistics, we do not have a good correlation between velocity consistency, ballistics, barrel harmonics, and long range precision.

We also do not have a really good way of measuring the internal volume of a case. Anyone who has tried it has had an air bubble escape during weighing with water or alcohol. Those little bubbles can and do affect the measurement of internal volume. If we cannot measure the expected causal factor of the cause/effect relationship, there is no need to go further in trying to correlate the cause to the effect.

If I were to try to isolate the case volume effect, I would do it this way:

1) Achieve a better level of precision in case volume measurement
2) Utilize multiple short strings of fire, in the best conditions possible, at 1000 yards to determine with reasonable statistical certainty if the case volume is detectable in POI
3) While conducting #2, utilize a doppler radar unit to plot velocities.

Assuming a well tuned rifle (like a 1000 yard benchrest rifle), we could then determine if the case volume affects POI and if the readings on a chronograph affect POI.

Until this is done, I'm sorting cases, weighing charges to the granule, and sorting by seating pressure.

OK, enough rambling... Just one shooter's opinion.

 

[Shaggy357]

Loading books are full of data but they couldn't even come close to addressing a question like this. They lack the specifics you are seeking due to the plethora of variability out there. Primer heat, powder burn rate, case length case volume, powder/air mixture within the case, bullet seating depth, humidity, temperature, altitude, direction of shot, etc etc etc......

Heard a story once about a couple that shot pistols competitively. They both used one specific gun in the same competition event. Same load. Everything exact except the shooter. Velocity difference between husband and wife was almost 150fps. Huh? How could everything be the same, except the shooter, and have that much difference in velocity. One of the shooters came down to shooting position so powder was nearer the primer. The other shooter brought the gun from down low up into position so the powder was closer to the bullet. Location of the powder within the case made the difference in velocity.

There have been articles on similar subjects, different brands of brass, same prep, and difference in case volume. Effect of different brands of primer on certain powders. Difference in neck turned cases vs not neck turned. Difference in bullet seating depth and velocities, pressures, accuracy, etc.

The effects of powder distribution within the case, case volume, voids, etc would be really hard because there is no one good, tried and true, method of controlling how the volume is used within the case. I have some shot and other specialty loads for some of my pistols. I have done enough research to prove that with my method of loading, I get better consistency with my loads in straight wall cases if I use a foam wad to keep the powder near the primer. Other folks have tried reduced loads in bottle neck cases using pufflon, pillow stuffing, and similar buffers to fill the void left when small amounts of fast powders are used. They may have worked in some instances, and not in others. One brass, powder, primer, bullet, and filler may work in one experiment, change a component, and results change. No tried and true method of research on case volume to my knowledge.

Steve

 

[Ggmac]

Check out , "Conservation of momentum " It may apply

 

[Sheldon N]

Velocity difference between husband and wife was almost 150fps. Huh? How could everything be the same, except the shooter, and have that much difference in velocity.

Easy, the wife limp-wristed the gun. Pistol is much smaller than a rifle so how much resistance against recoil the gun experiences could dramatically affect velocity.

 

[passion4shootin]

Hate reviving old threads but here goes.
Just shot a new brass of 6x47
( from 6.5x47 ). Used a Doppler chrono.
Want to find out my starting velocity with
my recently acquired rifle. Anyway, out of 5 shots my lowest/highest velocity were
3066/3087 with 40.2 grains H4350 using Bart’s Infinity 105. Anyway, I decided to
Check the case weight, case volume, and speed of the 2 cases.
Case 1: 3066 velocity
160.945 grains wt
47.720 grains water vol. capacity

Case 2: 3088 velocity
160.480 grains wt
47.860 grains water vol. capacity
1. Out of curiosity, this .465 grain difference, how much brass would that be.
2. This 0.140 grain difference in volume,
How big is it.

I place a piece of brass on the platen from turning my case. It weighed 220 grain.

In the second picture, I managed to get 145 grain drop of water on my platen and placed a grain of H-4350 for size comparison.

Interesting. For one, we can’t really assume that the extra brass has anything to do with volume, it may be in the rim.
Secondly, the case volume will be affected by bullet seating, so that ogive to base length consistency will play into it.
Thirdly, I don’t know what to make with the results of my weighing, volume relationship. It’s
Not what I expected. No easy solution. I’m back to culling shells according to their impact at 1K.