Case volume question

[cosmic creepers]

Hi. New to the forum, read a ton of different threads, but wanted a chance to reply to a group of knowledgeable guys about this.

Does anyone have a formula that is somewhat reliable for applying my same accuracy load to brass of different volume?

I've inly been able to do this once and I think was by accident, was a load for .223 that I was able to get three different loads in three different brass that shot under half inch, but can't seem to figure out the adjustment rates for .308 etc.

Thanks!

-P

 

[Ned Ludd]

Are you asking how to normalize charge weight to different case volumes? Simply reproducing the original velocity in the new [volume] brass is the best way I know of to accomplish that. It will usually get you pretty close. Generating a reliable one-size-fits-all formula to calculate the new charge weight based on the case volume differential might be a little more challenging. I would suggest using reloading software such as QuickLoad or Gordon's Reloading Tool, which are designed, in part, to accomplish exactly what you're asking. Even such programs are not perfect. The larger the adjustment, the more likely the resulting change in the load will not end up providing exactly what is predicted. Nonetheless, they can be very useful tools and will usually get you close.

 

[Straightshooter1]

Hi. New to the forum, read a ton of different threads, but wanted a chance to reply to a group of knowledgeable guys about this.

Does anyone have a formula that is somewhat reliable for applying my same accuracy load to brass of different volume?

I've inly been able to do this once and I think was by accident, was a load for .223 that I was able to get three different loads in three different brass that shot under half inch, but can't seem to figure out the adjustment rates for .308 etc.

Thanks!

There's more variables at work than just case volumes. There's no good "formula" that you can apply, though an app like QuickLoad or Gordon's Reloading Tool can take a lot of those variables into account to get one pretty close when volumes change. On a very simple simple basis, depending of the powder being used and the size of the cartridge, one an add or subtract a grain or two or fractions to test and match the velocity that works for you.

 

[JFrank]

A node can and will move slightly with every component change.
Case volume varies ~ pressure varies.
A simple charge ladder will give you all the information you need.

 

[William Vogel]

As long as the neck tension is equal ( not the diameter difference between sized/seated) and you can equalize pressure between each different brass, you should be able to get nearly exact results. The tension is going to move challenging than the volume.

 

[rwj]

As long as the neck tension is equal ( not the diameter difference between sized/seated) and you can equalize pressure between each different brass, you should be able to get nearly exact results. The tension is going to move challenging than the volume.

If not via diametral change, how does one measure neck tension?

 

[mgunderson]

Curious why someone would want to use 3 different brass volumes? Is this one rifle? How much of a volume difference? What range are we playing at? ......why is the most important.

 

[William Vogel]

If not via diametral change, how does one measure neck tension?

There’s no real way to do it. That’s where the mystery lies with using different brass. You could use a pressure transducer to measure seating pressure and graph the data to compare. Other than that, I don’t know.

 

[cosmic creepers]

Curious why someone would want to use 3 different brass volumes? Is this one rifle? How much of a volume difference? What range are we playing at? ......why is the most important.

Because for Remington Peters brass, for example, I have brass ranging from 162[point-something] to 168 grains in weight, and varying from ~54 to 56 grains of water in fired case volume.

I promise you, it isn't by choice. I just can't afford to only use brass that I developed my load with, but it is a narrow node and I can't afford to do a lot of things. So I'm trying to see if someone a lot smarter than me has a formula for adjusting charge with case volume

 

[cosmic creepers]

I just want to say thanks for the replies guys, I just ordered a couple hundred pieces of starline brass, and we will see if I get some consistency out of it. I appreciate the help!

 

[cosmic creepers]

Are you asking how to normalize charge weight to different case volumes? Simply reproducing the original velocity in the new [volume] brass is the best way I know of to accomplish that. It will usually get you pretty close. Generating a reliable one-size-fits-all formula to calculate the new charge weight based on the case volume differential might be a little more challenging. I would suggest using reloading software such as QuickLoad or Gordon's Reloading Tool, which are designed, in part, to accomplish exactly what you're asking. Even such programs are not perfect. The larger the adjustment, the more likely the resulting change in the load will not end up providing exactly what is predicted. Nonetheless, they can be very useful tools and will usually get you close.

That is exactly what I'm asking. I don't know exactly how to get the case space measurement required in those load spec programs. [I use p-max] because every time I try to doctor a case to measure water displacement it is just too erratic. I would need to find some volumetric for the bullet volume and use math to calculate displacement, which I just haven't done yet. So they get me [usually] in the ballpark, but I don't use for fine tuning.

I'm trying to see if guys have a predictable standard for taking a known accuracy node and staying in it with a case volume change.

Rifle shoots great with 46.5 grains of CFE223 at 2.805" with 46 gr H²O brass, but move to 43 grain H²O volume and she dumps.

I heard a feller mention 1 grain of charge for every 11 grains of brass weight, which sounds pretty logical from a safety standpoint, but there's got to be more to it.

 

[JFrank]

Rifle shoots great with 46.5 grains of CFE223 at 2.805" with 46 gr H²O brass, but move to 43 grain H²O volume and she dumps.

Not to be a wise guy but I believe the node moved, this is why I run abbreviated ladders when I change brass or any component. Targets never lie.

 

[DaveTooley]

Seems pretty simple to me. A volume weight change from 56 to 54 grs is a volume decrease of about 3.5%. I would reduce my load by 3.5%. But then again I probably wouldn't worry about.

 

[Jager]

As @Ned Ludd described above, if case volume is the only thing changing, whether because it's a different headstamp or because of lot-to-lot variation, it's very easy to accomplish with QuickLoad or GRT. It's not exact to a gnat's ass, because a case volume change will change the pressure profile, if only very slightly. But if you had a good node to begin with, with a good model in either of those software programs you'll hit it on the first pitch.

I did that very thing last winter with a .30 BR benchrest gun. 6 BR Lapua brass (with which .30 BR is formed) being hard to find, I bought a case of Peterson 6 BR. The Peterson is fine brass, but it's slightly thicker than Lapua and has correspondingly less case volume. It didn't take long in QuickLoad to find that I needed to reduce charge weight 0.3 gr when using the Peterson.

You need to confirm at the target and with a chronograph, of course.

Or, if you don't have the software you can just use a chrono and throw charge weight darts at the wall until you figure it out.

Or, you can just do a quick workup and run a charge weight ladder.

I don't think you're going to find a simple equation or algorithm which works.

 

[cosmic creepers]

Rifle shoots great with 46.5 grains of CFE223 at 2.805" with 46 gr H²O brass, but move to 43 grain H²O volume and she dumps.

Not to be a wise guy but I believe the node moved, this is why I run abbreviated ladders when I change brass or any component. Targets never lie.

Yes, that's the entire reason for the question, to see if anyone has an alternative to shooting a ladder, which even an abbreviated ladder could cost me upwards of 40 dollars of components and fuel.

Searching for a guy who knows more than that on the subject of pressure changes and charge weight to case volume relationships.

 

[cosmic creepers]

I ap

As @Ned Ludd described above, if case volume is the only thing changing, whether because it's a different headstamp or because of lot-to-lot variation, it's very easy to accomplish with QuickLoad or GRT. It's not exact to a gnat's ass, because a case volume change will change the pressure profile, if only very slightly. But if you had a good node to begin with, with a good model in either of those software programs you'll hit it on the first pitch.

I did that very thing last winter with a .30 BR benchrest gun. 6 BR Lapua brass (with which .30 BR is formed) being hard to find, I bought a case of Peterson 6 BR. The Peterson is fine brass, but it's slightly thicker than Lapua and has correspondingly less case volume. It didn't take long in QuickLoad to find that I needed to reduce charge weight 0.3 gr when using the Peterson.

You need to confirm at the target and with a chronograph, of course.

Or, if you don't have the software you can just use a chrono and throw charge weight darts at the wall until you figure it out.

Or, you can just do a quick workup and run a charge weight ladder.

I don't think you're going to find a simple equation or algorithm which works.

I appreciate that insight. What you're saying though is that as velocity with a given poder/bullet combination changes, I should be able to guess at velocity being the indicator of my accuracy node? Because that I can guess at with algebra and get pretty damn close at least to have a three charge weight ladder instead of shotgunning a target with a bunch of guesswork.

 

[linebaugh]

Yes, that's the entire reason for the question, to see if anyone has an alternative to shooting a ladder, which even an abbreviated ladder could cost me upwards of 40 dollars of components and fuel.

Searching for a guy who knows more than that on the subject of pressure changes and charge weight to case volume relationships.

Keep searching beother. Good luck.

 

[Jager]

I appreciate that insight. What you're saying though is that as velocity with a given poder/bullet combination changes, I should be able to guess at velocity being the indicator of my accuracy node? Because that I can guess at with algebra and get pretty damn close at least to have a three charge weight ladder instead of shotgunning a target with a bunch of guesswork.

Yes, that's what I'm saying.

When we work up a load and come upon what is typically described as a "node," we've happened upon a harmonious pressure profile / velocity / burn time in barrel / muzzle pressure... which results in tight groups downrange.

Assuming an accurate chrono, velocity is a good proxy for pressure. And assuming we're just talking about a very small change - like it would be with a case volume difference - those other factors won't change much. And so the chrono works for quickly finding our way back into that harmonious relationship.

The caution is that while velocity works great as a proxy for the entire pressure profile in a small-change example like this, and while velocity is always correlated with pressure in a general sense, we can't always say with certainty that just getting back to the velocity of our previous load makes us golden. For example, if we changed powder, to something faster or something slower, we might easily get back to the same velocity... only to find that our accuracy isn't nearly as good. That's because the pressure profile is different. And that pressure profile matters.

 

[Ned Ludd]

I appreciate that insight. What you're saying though is that as velocity with a given poder/bullet combination changes, I should be able to guess at velocity being the indicator of my accuracy node? Because that I can guess at with algebra and get pretty damn close at least to have a three charge weight ladder instead of shotgunning a target with a bunch of guesswork.

One can certainly "ballpark" an estimate, and it will likely work pretty well. The downside to that is that making a good estimate usually requires empirically determining velocity with a given charge weight in a few different case volumes so that the estimate of how velocity changes with respect to case volume can actually be made. Alternatively, reloading programs such as QuickLoad and GRT can make these predictions given solid inputs. However, one problem with any of these approaches is that they're typically based on the assumption that the effect of chagning case volume must always be a linear response. I can assure you that that is not always the case.

An example of this is that combustion of powder is a chemical reaction. We use a defined amount of reactant (i.e. powder), in a defined reaction chamber or pressure cell volume (i.e. the case expanded to match the chamber space), at a given temperature, so on and so forth. The problem lies in that the chemical reaction (i.e. powder burn rate) will increase as pressure increases. Thus, as pressure increases due to increased charge weight or decreased pressure cell volume, the powder burn rate can also increase, leading to pressure increasing at a rate even faster than would be predicted from a simple linear response to charge weight or a change in case volume. In other words, the response is a "curve", not a straight line. This effect will be even more noticeable as one approaches MAX pressure. As long as one uses previous experimental data or one of the reloading programs to make predictions within a certain range of charge weights or case volumes, the "curve" will effectively be close enought to linear within that limited range that the estimates are pretty good. The farther away one tries to predict such changes, usually the farther away from "reality" the prediction will be.

So a lot of this depends on how much effort you want to put into this. In your position, I might consider conducting the following test: sort your brass into three volume groups - low/medium/high. Keep track of the values you used to sort the cases into each group (i.e. the group sorting parameters). Load up a few rounds of each holding all else besides case volume as close as possible (i.e. use the same primer, charge weight, bullet, seating depth). Then shoot each group and record velocity. With only minimal effort, you should be able to generate your own "formula" based on the velocity versus case volume data that will provide a reasonable estimate within a case volume range that is wide enough to be useful for you as you move forward with the reloading process.

 

[DocBII]

CC,

Lapua used to say to decrease the powder charge by 0.1 gr. for every grain of increased case weight, up to about 10 grains. You could increase it by the same amount for every grain lighter. They also advised to start low and work up, which is always a good idea.

YMMV,
DocBII