Does case interior volume have an effect on velocity? i.e. is weight batching brass worth it?

[PeteFox]

Alan I did something very similar almost 3 years ago here >> viewtopic.php?f=5&t=11906&start=45#p94781 using Lapua 308 brass
I came to the conclusion that there was no relationship between case weight and volume.

I have made many mistakes reloading including omitting the powder and getting a projectile stuck half way up the barrel. So now every loaded round goes onto a small scale before boxing so I don't make that mistake again.
So I continually see a weight variation in my ammo, the bulk of which can only come from the case and the question of the effect of the weight variation always presents itself.
I'm on a mission to classify the variation into the 'it doesn't matter' basket and I thought those believers might put up something to challenge my view.
Pete

[Aus9914]

Pete,

Understand your question is theoretical, but if you are going to accept the lot weight tolerance of the case manufacturer on the assumption human error hasn't found it's way into the mix why bother constraining your powder load to 0.02 grn weight tolerance.

Al

[PeteFox]

Pete,

Understand your question is theoretical, but if you are going to accept the lot weight tolerance of the case manufacturer on the assumption human error hasn't found it's way into the mix why bother constraining your powder load to 0.02 grn weight tolerance.

Al

There's an easy (long) answer to that.

In the test I did, the theoretical assumption made was that the outside of the cases were uniform and that all of the excess brass displaced internal volume - a worst case scenario or a perfect world depending on your viewpoint.

My previous post on this (related) topic ( viewtopic.php?f=5&t=11906&start=45#p94781 ) established that case weight and internal volume weren't corrrelated , So it was nonsense to batch cases based on brass weight and assume that you were in effect batching by internal volume.

This is because the thought process that defined the methodology is arse about and every test I have seen leaves out the most critical step.

In order to batch cases by weight to infer internal volume, you would need a large number of fired, cleaned and trimmed to length cases. Then it would be first necessary to measure their external volume and discard those cases that display deviations from a predefined 'acceptable' variation. This is because assuming the excess weight is displacing internal volume when it is not introduces additional errors into the mix
Measuring the external volume would be difficult and need precise volumetric measuring equipment (liquid displacement) or some sort of laser scanning trickery. All way too expensive and time consuming.
Once that was done then yes case weight batching would infer internal volume.

But ... what do you do when cases need trimming and some need trimming more than others? That machined off brass comes from somewhere and affects volume. Do we than have to start batching within batches.? Too far down the rabbit hole for me.

This sort of mucking about is probably doable with a bench rest rifle where a small number of cases is normal, but I run 300 cases per rifle in F class x 2 rifles. As a guesstimate I would probaby have to buy 3 times as many as needed (1800) to get to batches of 300. That's a head spinning amount of weighing and measuring to do for minimal gain.

With respect to powder weighing down to 0.02gn- it is easily esablished that variations in powder weight will create velocity variations.
Being precise won't induce any errors - it won't make accuracy worse.
It all happens automatically with minimal assistance from me, it is what the machine is designed to do.
It has a certain confidence building effect.

3 pieces of 308 brass, all made to SAAMI spec, all having a different external volume

Screenshot_20230107_094233_Photos.jpg

Pete

[AlanF]

Pete,

Understand your question is theoretical, but if you are going to accept the lot weight tolerance of the case manufacturer on the assumption human error hasn't found it's way into the mix why bother constraining your powder load to 0.02 grn weight tolerance.

Al

Al,
I would add to Pete's post that choosing what things to do to improve accuracy is not all about comparing gains between them. Any one thing you do should be decided on a cost (time and money) vs benefit basis in its own right. The reason is that something causing a large error will not necessarily "absorb" a small error from another source. Occasionally they will produce an error in the same direction i.e. the errors compound. Velocity variation has a large number of possible causes. The more causes you ignore as not significant enough to worry about, the greater will be the statistical outliers where purely by chance several things all go in a particular direction and you'll get a muzzle velocity extreme (high or low). This is one of the reasons that Std Dev is generally regarded as a better indicator than Extreme Spread for measuring Muzzle Velocity variation.

[Frank Green]

I did a few times weigh sort cases to some extent but don't do it at all. I feel it's a waste of time because the interior case volume of the case is what makes a difference.

I've even have had case manufacturers tell me and I will quote..."A slight difference in how the extractor cut was made in the rim of the case from one case to the next will change the weight of the case but it has nothing to do with the volume of the case and the effect it has on powder burn/velocity."

So that's just one variable.

I will tell you this as well and most of you have seen it as well. I've seen it but only wondered about it. Another ammo/bullet maker told me/confirmed my suspicion....that a virgin case vs a once fired case...will have an effect on ES and SD's which as we all know effects velocities. The once fired case when reloaded....the ES/SD's will drop. I forget what the percentage of drop was. I could ask again. The once fired case basically fireformed/blew out any irregularities internal to the case it self.

I've seen it myself also with like 284win. I've had old 284 cases Winchester made from the late 1980's that weighed the same as a batch of 6.5x284 Lapua brass and I've had 284 Win. brass made by Winchester back around 2010. The 2010 made brass was significantly heavier (like 20 grains or so heavier...don't remember the exact number but it was a big difference) than the old Win. brass or the Lapua brass and I had to back off the powder charge on the newer Win. brass.

So I believe internal case volume if you want to call it that has more of an effect more on velocities than a slight difference in case overall weight. Assuming all the other variables are equal. Same powder lot, charges, bullets etc...

Later, Frank