Revisiting: sorting cases by weight

[Mikecr]

Unless I'm missing something.

You're missing something incredibly significant to internal ballistics: initial confinement.
And much of what matters to our endeavor(accuracy), has occurred before the point of your 'conclusion'.
Also, this measuring is not for an average. It's to match every single case & result.
You don't reach this, and know it, with shortcuts/assumptions. So if your intent is to match volume, then measure and match volume.

 

[ntsqd]

Define "Initial Confinement" because I'm not sure of how you're using it.

I don't think that I am measuring for an average unless you're saying that all of the cases used should weigh as exactly the same as-is reasonably possible.

 

[misterc01]

After weighing and all sorts of other manipulations - I found if it chambers and fires, it worked just fine. Consistent powder, seating depth and concentricity seem to make a larger difference.

 

[ntsqd]

I suspect that you're right. I recently got two completely new to me calibers and I'm looking to "put my best foot forward" so to speak. I'll likely have lots of bigger challenges before this technique bears any measurable results, but I want to set myself up for success. I have a lofty goal for both, shooting an order of magnitude greater distance than I have loaded and shot in the past. Couple that with I haven't reloaded much in 20 years and I've got some catching (& tooling) up to do.

I was really hoping that someone from the "measure case capacity with water" camp could illustrate why weighing the empty cases isn't enough.

Assuming my hypothesis is correct, knowing the case weight, case dims, and the brass density I could easily calculate each case's volume. Excel makes this reasonably possible to do for every case though I'm unlikely to go to that extent. Simply sorting by weight should yield a set of cases with very similar capacities. No two things are ever identical and I suspect that BR comps are timed (at least those that I've shot) so that the competitors don't load a new round in the same case for every shot.
The tighter the tolerance for the case weights, the closer the capacities will be. Given that brass is denser than water I expect that a small variation in case weight will equal a much larger variation in water volume, so the case weight tolerance needs to be proportionately smaller to equal the same accuracy.

I'm guessing that case capacity is measured by using a graduated beaker. Fill to a meniscus and note the difference in the titration beaker? How consistent can that be, and what is the accuracy of the measurement?

 

[Mikecr]

With our brass downsized to fit in our chambers there is the difference potential between initial and expanded area of containment, and also of confinement.
The downsized case provides a lower area of containment (initially).
It is also less confining (initially) in that it will expand with case resistance only -until reaching the chamber walls.

You might think that a sized smaller case would increase load density for a given charge and this would raise pressure. Very possible, depending on chamber fit.
With a tight chamber fit, relative confinement is higher, and if a smaller area of containment is set to that tighter fit, then pressure will go up.
But many folks run with loose chamber clearances. This acts to reduce the pressure peak. To flatten it. Apparently, a significant portion of a pressure peak is consumed to expand cases to chamber walls. Then, I imagine, when the case hits higher confinement of the chamber walls, and depending on bullet travel timing, pressure could rise again.

The jest of my concern here is that capacity affects pressure peaks. Therefore, variance of capacity could vary my pressure peaks. So I don't care what cases weigh, I want the same capacities, and same fit & form.

Next, you need to understand all that is behind achieving this.
When you do, you'll know that separating cases by weight instead of capacity doesn't really help, and can actually be detrimental to your intent.

 

[ntsqd]

Odd as it might sound I don't really care what the cases weigh either. I am not seeing how measuring case capacity in H2O tells you anything other than that. It won't indicate thick base, thin walls, it won't indicate thin base, thick walls.

To be sure, neither will weighing them. It will only tell you what they weigh.

Say all of the cases in question are at the same form, once fired, thrice fired, brand new, brand new and FL sized, doesn't matter what they are so long as they all are that. Arguably their exterior shape would be the same. Suppose you measure case capacity in H2O and sort them accordingly. It should directly follow that those with the most H2O capacity will weigh the least and those with the least H2O capacity will weigh the most. More room for water means less brass within a fixed set of exterior dimensions.

 

[Mikecr]

There is correlation between weight and capacity with some lots of some cartridges. I've seen it, and I've seen otherwise.

Matching weight itself can be difficult given your case preps. For me, I'm setting primer pocket depths, turning necks, chamfering mouths, trimming settled necks w/resp to or within 5thou of chamber end, which is hopefully by 3rd FFing, re-chamfering.
I've invested money/time in the prepped cases before I could cull by weight, and then I would wonder with each what their capacity really is anyway. Would 20 cases separated as matching in weight actually match in capacity? Would there be a few that don't? Are any of 5 pulled out for weight departure actually matching capacity of the 20 group?
Hell I might as well just measure capacities. It's the only way to know.
I'm also calibrating QuickLoad for gun, which I need accurate capacity for anyway.

What varies capacities of cases weighing the same, even if webs/heads match in form, and all preps completed, is case spring backs. They're tapered in thickness from webs to mouths, varying of course, which varies spring back from up/down sizing (reload cycles). The more you size this variance the greater the variance grows. Throw partial length annealing in there, maybe it isn't precise,, that won't help in capacity matching. Using the brass at different pressures during load development, leads to more/less sizing of that brass in the same dies. Different hardening for spring back changes.

I've been matching capacities a long time. There is no shortcut, and I toss a significant percentage of brass with it. I use custom chambers and dies, and I choose smaller improved cartridges that are viable for this. 260AI over 6.5x284 (for example), and forget about it with something like a 264wm, or a 30-06..
The only reason to match capacities is for long term accuracy, and that includes a plan that allows long term use of brass (which you would invest so much into).

IMO, the only value in weighing cases is in finding problems. X lot weighs way more/less than Y lot. 7 out of 4,000 had missing flash holes or mutated case heads. If you write all these weights down, you're still a long way from grouping anything by capacities. So if not willing to go through the efforts of actual capacity measure & management, maybe it's not viable to do so, you can still weigh cases.
Just don't assign excess credit for that.

 

[misterc01]

Measuring a case with water is just like measuring cylinder volume in an engine - it gives you the ACTUAL displacement of the engine, or capacity of the case. That can be valuable information or not - depends on the reloader. I have tried it and got a nice bell curve on weights, but realized I needed to do more in optimizing the cases to ensure they were dimensionally the same - length, primer pocket sizing, etc. So I will give it another shot (pun intended), but to date not prerequisite to my ammo prep.

 

[ntsqd]

I am trying to use simple case weight as a preliminary case sorting method. Weed out the real outliers before any of those case prep chores were performed on them. With something like Quickload needing the case capacity in water I don't see a calculated case weight being a viable substitute. For that to work you will need to supply what it asks for.

With brass being 8.5 times denser than water I can see how small changes in the brass can have an effect on total case weight, i.e. the various case prep chores can have an unexpectedly large effect on case weight. Obviously case size will be important as to the overall effect.

Becomes kind of a chicken vs. egg problem. Those chores are to make them more uniform, yet why invest in all of that work in a case that is an outlier to start with. Yet if you don't invest in the work, how do you know that the reason it is an outlier is because it has excessive primer hole flash?

 

[ShtrRdy]

I've been doing a little experiment with some new Lapua brass for the .308 Win. I had weighed the 100 cases in the box and measured a spread of 3.2 grain. I thought this was a little large for Lapua so I wrote the customer service and they told me that their manufacturing strives to obtain a close case capacity and the case weight can vary.

So I took three cases on the low weight end and three cases on the high weight end and fired them. Then I measured the fired case weight and filled it with water. I then measured the case plus water weight and subtracted off the empty fired case weight.

The first time I did this, ( new brass ), I came up with the following:
Lite brass H20 weight = 55.8, 55.6, 55.7
Heavy brass H20 weight = 55.4, 55.5, 55.5

I repeated this test using the same pieces of brass but this time they are once fired.
Lite brass H20 weight = 55.7, 55.6, 55.7
Heavy brass H20 weight = 55.3, 55.4, 55.3

The second time I performed the test I measured the velocity of the load I was using. This comprised of a CCI Large Rifle primer, 175 Sierra Matching and 41.5 gr IMR-4064.
Lite brass load velocity = 2576, 2590, 2599
Heavy brass load velocity = 2603, 2601, 2598

There is some difference between these cases but aside from the velocity of 2576, not too much. It's interesting the velocity measurements on the "heavy" cases were very close to each other and that there was a bit of a spread in the velocity measurements on the "lite" cases.

I don't know if this relationship would apply to other brands of brass. I thought I had run experiments in the past by just loading and firing "lite" and "heavy" brass loads across a chronograph and would see a significant difference in velocity.

 

[Mikecr]

I wrote the customer service and they told me that their manufacturing strives to obtain a close case capacity and the case weight can vary.

I don't know if this relationship would apply to other brands of brass.

To convert what Lapua is telling you: they focus on the case forming and not so much about weight. That they don't know weight correlations from lot to lot, nor case to case.
Brand-to-brand correlations would be impossibly inconsistent.

There are no shortcuts to the reality that WE create our cartridges (locally).
So when something really is one way or another, it's because we made sure it is that way.

 

[Don A Parsons]

I go by case volume only.

Find a batch of cases of same volume then run with it.

 

[Buster Hemlock]

Hopefully this isn't too long winded or confusing. When this post first popped up it got me thinking about some nosler 6.5 creedmoor brass I've been getting from a guy at the range that doesn't reload. I've got close to 900 pieces off him piled up for use in a AR-10 and they are from 7 different lots. They are still in the boxes, so I decided to sort some from different lots by weight and then measure capacity to see if the lots with similar weight could all be combined. I pulled six cases, three cases each from two separate lots that weighed within .2 grains, 161.1-161.2 gr. of each other. This took a while sorting through a lot of brass to find lots with similar weights that I thought may be worth combining. Within the individual lots capacity was within .1 gr. H2O for the cases I was comparing. Yes this is a small sample size. But between the two lots with virtually identical weight the difference was .4 gr. H2O. I then took the heaviest and lightest from the two separate lots and came up with a case weight difference of just over 3 grains. I measured internal capacity of the cartridges on the extreme ends and had a H20 capacity difference of .5 gr. H2O from one lot and .6 gr. H2O from the other. To me it seems that weight sorting within a lot is probably a reasonable way to ascertain internal volume. Mixing lots may not be advisable as it would lead to larger variation of internal capacity even though the case weights are similar. I threw my load info into Quickload and found a .5 grain variation of internal capacity corresponded to a theoretical 15 fps difference and 1.1 grains difference corresponded to 33 fps. So I guess it comes down to what your looking for but if your trying to shrink SD and ES I'd spend some time doing some experimenting and deciding for yourself what's worth your time. I'll be keeping the lots separate and weight sorting to minimize variation. I'm probably going to find 50 cases within 1.0 gr. case weight and take the time and test capacity and see if sorting by case weight holds up for myself.

 

[tbrice23]

I am trying to use simple case weight as a preliminary case sorting method. Weed out the real outliers before any of those case prep chores were performed on them. With something like Quickload needing the case capacity in water I don't see a calculated case weight being a viable substitute. For that to work you will need to supply what it asks for.

With brass being 8.5 times denser than water I can see how small changes in the brass can have an effect on total case weight, i.e. the various case prep chores can have an unexpectedly large effect on case weight. Obviously case size will be important as to the overall effect.

Becomes kind of a chicken vs. egg problem. Those chores are to make them more uniform, yet why invest in all of that work in a case that is an outlier to start with. Yet if you don't invest in the work, how do you know that the reason it is an outlier is because it has excessive primer hole flash?

I think we are trying to ultimately achieve the same barrel time from shot to shot.
I think consistent neck tension (annealing), sorting bullet bearing surface and/or bullet weight sorting and exact powder charges may make more of a profound influence on barrel time than measuring minute differences in case volume BECAUSE until the bullet has left the muzzle the space from the end of the flash hole to the end of the barrel becomes the actual chamber.

Powder doesn't completely burn inside the case! It burns all the way down the barrel.