Case weight vs internal volume

[benchracer]

For those of you who weight sort brass (of the same headstamp), how closely have you found internal case volume to correlate with case weight?

Put another way, among a group of cases that weigh the same, how much have you found the internal volume to vary?

 

[CB11WYO]

Tag...

 

[Mikecr]

1. H20 capacity correlation with case weight shifts around with cartridges & lot #s of it.
2. It also shifts around with your ability to consistently measure H2O capacity.

I've seen patches of correlation, like with 2-3 bags out of 5-10. I've found flash hole shavings in a few cases. But for the most part I've accepted that the only way I'll know capacities match is to actually measure this(H2O).

 

[Deleted member 46119]

My method and belief:
The cases have to fully prepared and fired, only then will they be formed enough to make a meaningful measurement.
Once fire formed, the cases should be cleaned of internal residue then the volume data can be gathered.
There will be a correlation between weight and volume during the firing process.
More data is better than less.

 

[Mikecr]

You're right the cases do have to be fully prepped and fully fireformed to measure H20 capacity.
No need to clean cases beyond normal vibratory(the carbon film is fine).

It's initial volume/load density that sets up the peak in pressure curve, much like neck tension does.
By the time brass displacement variance in your chamber would come into play(expansion ratio), it is completely irrelevant. Much of what really matters to tune has well past by then.

But this does bring to point the fact that brass weight itself is meaningless to initial volume. You can crush those cases with a hammer to near zero volume -and they still weigh the same..
You can size down half of same weight cases enough to show differently across a chrono than the other half, because with the same charge, load density is different between the two volumes.

There is a work-around for this other than managing volume: Extreme pressure loads.
But this is only viable with little underbore cartridges like a 6PPC/30BR (not with most hunting capacity cartridges). Here, the pressure peak is in a return diminished region(in a sense) so that variance in it means 'relatively' less to tune. Load a 6PPC to normal pressures, hunting cartridge pressures, and it don't shoot like a competitive 6PPC anymore. It shoots more like a 6br or 223.
A 6.5x47L & 123gr bullet is pretty much the upper extent for this approach, and only with a magnum action to get enough barrel steel around the chamber.

 

[Deleted member 46119]

Mike,

I feel like we've been here before but perhaps it was someone else.

Case volume during the firing process and brass weight does have a correlation. A direct correlation.

It's physics. The brass consumes chamber volume. More brass less residiual volume to fill during the expansion phase of the firing sequence.

The less chamber volume the less powder needed to reach the same pressure but also a different timing for bullet release pressure. Different pressure form and duration.

If all this were 'irrelevant' then all loads would behave the same in different brands or manufacturing lots of brass and we know this ain't so. Point of fact some loads are fine in one brand but are dangerous in another. One of the chamberings I load has 10 grains difference between brands of brass.


It doesn't take that much pressure to reach the brass stretch and yield point or case necks won't need to be sized, cream o'wheat fire forming wouldn't work, ect..

Yes I simplify, why, because I can without being misleading or incorrect.

 

[Mikecr]

Look at it as INITIAL CONFINEMENT. Very similar to neck tension affecting initial confinement and changing the pressure curve enough to affect tune.
Initial confinement can change load density and speed or slow powder burn rate.

You put a little powder in a case without tamping & fire it, you get WHUMP.
Put a bit of COW, or even a single piece of toilet paper on top of the powder & fire it, you get BOOM.

A load density test:
Load two equal capacity rounds to about ~90% load density.
At the range, point the barrel straight up, load one of the rounds, carefully lower the gun to a rest, and fire across a chrono.
Now point the barrel down, load the other round, carefully raise it to rest and fire across a chrono.
You'll likely get abnormal ES between them, even though the cases match in capacity & regardless of case weight or it's chamber displacement.

Now imagine two equal weight cases, one FL sized every time, and the other never body sized at all. These cases will measure different in volume, and with the same load they will produce different muzzle velocities. It's not a lot of MV difference, you might not even see it with a cheap chrono, but it's there. And the peak pressure curve is also affected and your grouping results could change if these cases are mixed together in a string.
It's similar to mixing different brands or lots of brass.
And yes, different brands of brass present different capacities(volume).

 

[benchracer]

It's similar to mixing different brands or lots of brass.
And yes, different brands of brass present different capacities(volume).

Observing the shot to shot differences in some mixed case plinking ammo I had loaded, while playing with my Magnetospeed chrono, is what started me thinking about this. Just separating the plinking ammo by headstamp had a HUGE effect on ES and SD.

Taking actual volume measurements to refine my QL calculations got me to thinking about the subject even more.

Then, I started running some numbers through the JBM ballistic calculator to see how much vertical spread resulted from small velocity changes at 600, 1000, and 1500 yards. Not much at 600. Quite a bit at 1000. A LOT at 1500.

Now I am starting to understand why long range shooters do things like sort bullets and brass. It looks like consistent case volume is pretty important in keeping ES and SD numbers low. I just wonder if weighing cases yields accurate enough control of case volume to provide the level of consistency needed at 1000 yards and further.

I plan to experiment with sorting cases by weight vs actually measuring individual case volume to see how that translates into real world results. At some point, I expect to run into limitations involving primer and powder ignition consistency. I just don't know how far I will be able to narrow ES and SD before I get to that point.

I know I am re-inventing the wheel here. I am mostly trying to develop my own understanding of some of the variables at play, along with how and why they apply.

Thanks, fellas, for the discussion. I truly appreciate the opportunity to benefit from your experience.

 

[Barrelnut]

I plan to experiment with sorting cases by weight vs actually measuring individual case volume to see how that translates into real world results. At some point, I expect to run into limitations involving primer and powder ignition consistency. I just don't know how far I will be able to narrow ES and SD before I get to that point.

I know I am re-inventing the wheel here. I am mostly trying to develop my own understanding of some of the variables at play, along with how and why they apply.

Hey Benchracer, it would be great if you get a chance to post some of your tests results. Weight versus volume is always in debate, but I don't think I have found anywhere where actual testing supports the theories.

I agree that sometimes you just have to do this yourself to prove to yourself (take nothing for granted). I hope to test myself someday.

 

[benchracer]

Hey Benchracer, it would be great if you get a chance to post some of your tests results. Weight versus volume is always in debate, but I don't think I have found anywhere where actual testing supports the theories.

I agree that sometimes you just have to do this yourself to prove to yourself (take nothing for granted). I hope to test myself someday.

Like you, I have seen some discussion of the topic, but never seen any actual data, though I am sure that such data exists somewhere.

I fully intend to share my results as I generate them. I am still thinking through the design for the experiment. It will start with a measurement phase, involving case weight and volume, and progress to actual firing of each case. I hope to be able to generate specific enough data to track velocity changes relative to case volume and case weight. In the end, I would like to present every individual data point in a spreadsheet that can then be criticized and dissected by some of the more knowledgeable people here.

Like the other things I am working on, my schedule is such that I only have short windows to get anything done, so my progress will be slow and inconsistent. I will provide updates as I am able.

 

[Deleted member 46119]

OK Mike!

Now we are on the same page but can we simplify?

Case weight is always an indicator of volume.
Volume is not measurable until the case is in a known state. As in: fired.
New and fl resized cases will have a different volume than fired and that will effect load tuning.

 

[Mikecr]

Case weight is NOT a direct indicator of volume.
2 cases, same weight, dent one, it's volume is lower.
Even if you don't dent one, it's volume can be different.
Brass weight from web begin through casehead doesn't count for volume. And brass variance in springback affects volume.

Nothing leads to matching capacities except verification of just that.

 

[texastrophy]

I have sorted cases by weight and then by volume and have found EXTREME differences! It's very time consuming, but if you're seriously looking for consistent accuracy, check volume, not weight.

 

[MagnumManiac]

I have sorted cases by weight and then by volume and have found EXTREME differences! It's very time consuming, but if you're seriously looking for consistent accuracy, check volume, not weight.

+1
Totally agree with the above statement!

Cheers.
gun)