Case Capacity ( again)

[kiwijohn]

So I have ended up with an FX scale.... obviously my OCD has really kicked in with my reloading!
Also with the current shortages and being oppurtunistic I have gotten some random brass. This then begged the question will it perform the same as my Lapua SRP brass.
I ended up with some Peterson and Starline both in SRP configuration.

I began seeking the variances by weighing each style.

Lapua Av weight by grains 171.2 variation 0.2 Neck wall thickness 0.0145 - 0.015
Starline " " " " 172.6 " " 0.6 0.0140 - 0.015
Peterson " " " " 173.9 " " 2.9 0.013 - 0.0135

Not much in it really.
So then I went and dribbled water into them from a syringe and using an eyechometer to check the water level was flat and not concave or convex.

Water capacity of Lapua 55.52 grains
Starline 54.56
Peterson 54.62

Again not much in it so for my purposes I will consider them all even when reloading, I will continue to use the Lapua in my TR rifle and the other brass I will use in an FTR rig I am starting out with.

Here is the question about case capacity

Fired or unfired brass? the reason I ask is that if you measure fired brass shouldnt you also be taking into account the entire chamber out to the lands? As isnt this the true capacity during the detonation phase ?
I opted to measure resized brass as I felt this was more in line for the information I wanted, if the capacities had been of greater variation I would then need to take that into account with charge weights

 

[LVLAaron]

Fired.

 

[jelenko]

^^ The reloading tools all use fired brass capacity.

 

[Ned Ludd]

Here is the question about case capacity

Fired or unfired brass? the reason I ask is that if you measure fired brass shouldnt you also be taking into account the entire chamber out to the lands? As isnt this the true capacity during the detonation phase ?
I opted to measure resized brass as I felt this was more in line for the information I wanted, if the capacities had been of greater variation I would then need to take that into account with charge weights

As noted above by others - fired brass that has been expanded to fit the chamber. In a nutshell, you want to know the internal volume of a fired case, not the volume of re-sized brass, not the volume of the chamber, not the volume occupied by the bullet once it has started moving out of the case, just the full internal volume of the case itself. In terms of actual volumes, everything else would largely be guessing. There are good reasons reloading programs use the volume of fired brass, no more, no less. There is no need to re-invent the wheel here or make things more difficult than necessary.

 

[Straightshooter1]

So I have ended up with an FX scale.... obviously my OCD has really kicked in with my reloading!
Also with the current shortages and being oppurtunistic I have gotten some random brass. This then begged the question will it perform the same as my Lapua SRP brass.
I ended up with some Peterson and Starline both in SRP configuration.

I began seeking the variances by weighing each style.

Lapua Av weight by grains 171.2 variation 0.2 Neck wall thickness 0.0145 - 0.015
Starline " " " " 172.6 " " 0.6 0.0140 - 0.015
Peterson " " " " 173.9 " " 2.9 0.013 - 0.0135

Not much in it really.
So then I went and dribbled water into them from a syringe and using an eyechometer to check the water level was flat and not concave or convex.

Water capacity of Lapua 55.52 grains
Starline 54.56
Peterson 54.62

Again not much in it so for my purposes I will consider them all even when reloading, I will continue to use the Lapua in my TR rifle and the other brass I will use in an FTR rig I am starting out with.

Here is the question about case capacity

Fired or unfired brass? the reason I ask is that if you measure fired brass shouldnt you also be taking into account the entire chamber out to the lands? As isnt this the true capacity during the detonation phase ?
I opted to measure resized brass as I felt this was more in line for the information I wanted, if the capacities had been of greater variation I would then need to take that into account with charge weights

As mentioned, you'll want to use volumes of fired brass. If you want to go to some extreme, you can sort your cases by the volume to get more consistent performance. I've heard of some people actually doing that. NOT ME though.

Here's some of my .308 case volume data I've collected and keep on hand for reference:

Just today I finished doing some measurements on some new 6.5 PRC Peterson brass I just acquired (see below). It hasn't been fired yet, so they're all virgin brass and I'll measure their volume again after they've been fire formed. I did something a little different when weighing them this time in that I sorted the empty cases and chose 10 that had the same weight (far right columns) plus two outliers, one that was lightest and one that was heaviest. The 10 Peterson cases that weighed the same had the same volume (I didn't expect that). The two outliers I put in that last column with respect to their weight and their corresponding volume. All cases have the same OAL, including the Lapua cases that I trimmed to the same length for this comparison (Lapua cases were not sorted). My fired Lapua brass has an average case volume of 68.6 gr H2O) and looking at these numbers, I expect the Peterson brass will be very close to that, but we'll see.

 

[kiwijohn]

Ok thanks everyone for your input, I will measure again once I fire some. The comparison for Lapua to Peterson and Starline wont mean anything though as they will be from different chambers.

 

[mikecr]

The comparison for Lapua to Peterson and Starline wont mean anything though as they will be from different chambers.

You're just wasting time really.
But what is your end goal?

You may want your cases formed to stable in YOUR chamber, and matching in volume.
But even if you pull that off, which won't be easy with brass prior fired in different chambers, there are still the dynamic volume differences -unless the brass is the same alloy/hardness/thickness.
So for this endeavor, mixing brass is just what you should never do.

Matching case capacities is only one part of matching cases with same expansion energy.
If you're not familiar with expansion energies, it's seen as differing results with new -vs- fired -vs- sized.
It's energy consumption of peak pressure to expand cases hard into chamber walls.

For some combinations its a big deal, and brass that throws a shot should be thrown way further from the rest of your brass. Even with all I do to match everything, I cannot actually match everything.
So if I'm shooting true and a shot leaves the pack, I don't hesitate to toss that demon case.
I mention this so you can recognize that what presents as static readings on a bench is not cut & dry to dynamic field results.
Your target holds the final say

 

[6MMsteve]

You're just wasting time really.
But what is your end goal?

You may want your cases formed to stable in YOUR chamber, and matching in volume.
But even if you pull that off, which won't be easy with brass prior fired in different chambers, there are still the dynamic volume differences -unless the brass is the same alloy/hardness/thickness.
So for this endeavor, mixing brass is just what you should never do.

Matching case capacities is only one part of matching cases with same expansion energy.
If you're not familiar with expansion energies, it's seen as differing results with new -vs- fired -vs- sized.
It's energy consumption of peak pressure to expand cases hard into chamber walls.

For some combinations its a big deal, and brass that throws a shot should be thrown way further from the rest of your brass. Even with all I do to match everything, I cannot actually match everything.
So if I'm shooting true and a shot leaves the pack, I don't hesitate to toss that demon case.
I mention this so you can recognize that what presents as static readings on a bench is not cut & dry to dynamic field results.
Your target holds the final say

target trumps all yes

 

[RegionRat]

Fired or unfired brass? the reason I ask is that if you measure fired brass shouldnt you also be taking into account the entire chamber out to the lands? As isnt this the true capacity during the detonation phase ?

Both sets of numbers, virgin and unfired, are important if you load that way for a match.

Many folks have to use virgin brass in a match. (I even met a few club mates who only shoot virgin brass and sell or trade it off. Go figure.)

By the time you get serious at 600y and beyond you will test. By the time you get good, you will see the difference. Many times, you will need to use virgin brass just due to the logistics of life, so you will want to know the difference.

As for theory about internal volume and internal ballistics and the lands versus the case, etc., I would recommend that unless you plan on writing the code yourself, follow the instructions and enter the data as the program asks, i.e., the case capacity.

Models may or may not correlate to tiny details and the pressure differences near initial ignition isn't one of the things to sweat when looking at the results. Jam versus jump is on the order of 4ksi and not the end of the world unless your load is already in the red.

YMMV

 

[gunsandgunsmithing]

Not sure why it matters but if you want to know the capacity of a LOADED round, take a fired case and dent the case mouth to hold a bullet. Fill the case to about where you think the base of the bullet will be, plus a tad. SLOWLY seat a bullet. The excess water will come out of the case neck, but do it slowly or you could have a primer fly out dangerously fast, under pressure. That's why I'd NOT neck the case for a bullet.
Weigh before and after, and clean the dies and mess up. Done.

 

[kiwijohn]

For the foreseeable future lapua brass is unavailable in New Zealand. This is the reason to have to try what IS available
As stated the Lapua brass will continue to be used in the TR rifle the mixed headstamps of Peterson and Starline will go into the other rifle.
The FTR rig is under load development, my main reason to start this discussion was to gage opinion on the similarities of the 3 brass.
And the viability of using two brands through the one rifle in the interim considering they are very close in capacity.
I had overlooked the difference in material that may lead to variation in neck tension. I think I will just use the Starline brass and wait to see if more becomes available.
The other reason to change brands besides unavailability was it is a foolproof way to keep the brass matched to the rifle .
Thanks for the advice it is appreciated.

 

[RegionRat]

As far as Starline versus Lapua, etc...
A well screened contiguous set of brass that is all similar in capacity and also in firing history, even if it is all Starline, will likely give you better results than a mixed bag of different headstamps with various capacities and histories.

 

[kiwijohn]

Yes you are right, RR .I think perhaps my bargain brain overcome my common sense lobe, when I saw brass for less than $2 a case.

 

[mikecr]

May still be useful.
Could keep summer load brass and winter load brass in their respective brands.
I know that MY loads are different for these conditions.