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).
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.
“Ask, Listen, Learn, Grow”
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.
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.
“Ask, Listen, Learn, Grow”
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).