Case weight variability question

Mikecr

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In reloading it's often assumed that 'matching' a single attribute would be beneficial, based on no more than that.
But with some matters it's actually rare that a single attribute(by itself) holds credible meaning.
Sometimes you're best to simply measure the sum of all (the result).
Case weight falls under this situation

The function of case weight comparison serves to infer case capacities, -as a shortcut to actually measuring case capacities.
It takes little effort to consider the flaw in this approach though, as you could flatten a case with a hammer and it will still weigh the same, while it's capacity has gone to zero.
We don't need to match weight, but initial confinement and chamber clearances (2 things).
These should combine to a consistent form.

With a consistent form, the expanding cases should absorb energy to consistently shape pressure peaks. This, provided the cases exhibit the same modulus of elasticity. Brass attributes aside, case form to chamber clearances plays a big role in this.
It's why new brass shoots differently than fully fire formed brass. And this difference can be far larger than ~5gr of case weight disparity -spread throughout cases. Chamber clearances change where and what your optimum developed load will be.

Initial capacity and load density is important also, but not so much variance-wise. Mix a 5gr sliver of brass to the inner perimeter of a case while charging it, will it mean anything to muzzle velocity? Not with a hunting capacity cartridge. I doubt you could pick it out with a blind test.

So if fully formed to stable dimension cases match in H20 capacity, does it reduce this dynamic clearance abstract?
Not if you FL size cases a bunch. Here, just forget any measure & matching, as you lose it with the sizing and spring backs and trimming.
If you neck size only, it matters. Your load will be different and you can shoot & see the difference.
Low clearances reduces variance, but amplifies the affect of any.
Then on the flipside, high clearances are forgiving.

I run tight to fitted clearances on everything, so I match H20 capacities after fully fire forming.
I had measured new brass thickness, while checking it's thickness variance.
Before fire forming I had dip annealed ~3/4 down case bodies.
But ultimately, if a case throws a shot, I throw the case.
I don't care about it's brand, cost, weight, capacity, or all the prep efforts. It acted differently.
What makes it act differently is just not what you could predict.
 

Coyote Shadow Tracker

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Hey w
In reloading it's often assumed that 'matching' a single attribute would be beneficial, based on no more than that.
But with some matters it's actually rare that a single attribute(by itself) holds credible meaning.
Sometimes you're best to simply measure the sum of all (the result).
Case weight falls under this situation

The function of case weight comparison serves to infer case capacities, -as a shortcut to actually measuring case capacities.
It takes little effort to consider the flaw in this approach though, as you could flatten a case with a hammer and it will still weigh the same, while it's capacity has gone to zero.
We don't need to match weight, but initial confinement and chamber clearances (2 things).
These should combine to a consistent form.

With a consistent form, the expanding cases should absorb energy to consistently shape pressure peaks. This, provided the cases exhibit the same modulus of elasticity. Brass attributes aside, case form to chamber clearances plays a big role in this.
It's why new brass shoots differently than fully fire formed brass. And this difference can be far larger than ~5gr of case weight disparity -spread throughout cases. Chamber clearances change where and what your optimum developed load will be.

Initial capacity and load density is important also, but not so much variance-wise. Mix a 5gr sliver of brass to the inner perimeter of a case while charging it, will it mean anything to muzzle velocity? Not with a hunting capacity cartridge. I doubt you could pick it out with a blind test.

So if fully formed to stable dimension cases match in H20 capacity, does it reduce this dynamic clearance abstract?
Not if you FL size cases a bunch. Here, just forget any measure & matching, as you lose it with the sizing and spring backs and trimming.
If you neck size only, it matters. Your load will be different and you can shoot & see the difference.
Low clearances reduces variance, but amplifies the affect of any.
Then on the flipside, high clearances are forgiving.

I run tight to fitted clearances on everything, so I match H20 capacities after fully fire forming.
I had measured new brass thickness, while checking it's thickness variance.
Before fire forming I had dip annealed ~3/4 down case bodies.
But ultimately, if a case throws a shot, I throw the case.
I don't care about it's brand, cost, weight, capacity, or all the prep efforts. It acted differently.
What makes it act differently is just not what you could predict.

Hey watch watch you say about cats Jill and I rescue them.
Actually, there is a best way to skin a cat. _Well Bob or Couger is alright

Just a phrase - kidding
 

BrentM

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Case weight across the same head stamp is almost totally worthless. Most of the brass weight is the case head with some variability through out the case. What is important is internal volume.

I have done this exercise multiple times, take brass weigh it, mark it, and then do a h2o weight. There is very little correlation between the 2 as long as they are not grossly out of whack. As long as I am using same head stamp brass I've stopped messing with this crap and have had zero issue with low sd and good accuracy.

If you want to get into brass variance I'd take a hard look at the neck thickness and hardness. Maybe anneal, test thickness to ensure you are not getting huge variation in tension. Of course the other items are powder type, charge weight, primer, and seating depth. Being anal will pay dividends but I don't see it in the weighing of brass.
 

new2mud

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May 3, 2010
Messages
384
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In reloading it's often assumed that 'matching' a single attribute would be beneficial, based on no more than that.
But with some matters it's actually rare that a single attribute(by itself) holds credible meaning.
Sometimes you're best to simply measure the sum of all (the result).
Case weight falls under this situation

The function of case weight comparison serves to infer case capacities, -as a shortcut to actually measuring case capacities.
It takes little effort to consider the flaw in this approach though, as you could flatten a case with a hammer and it will still weigh the same, while it's capacity has gone to zero.
We don't need to match weight, but initial confinement and chamber clearances (2 things).
These should combine to a consistent form.

With a consistent form, the expanding cases should absorb energy to consistently shape pressure peaks. This, provided the cases exhibit the same modulus of elasticity. Brass attributes aside, case form to chamber clearances plays a big role in this.
It's why new brass shoots differently than fully fire formed brass. And this difference can be far larger than ~5gr of case weight disparity -spread throughout cases. Chamber clearances change where and what your optimum developed load will be.

Initial capacity and load density is important also, but not so much variance-wise. Mix a 5gr sliver of brass to the inner perimeter of a case while charging it, will it mean anything to muzzle velocity? Not with a hunting capacity cartridge. I doubt you could pick it out with a blind test.

So if fully formed to stable dimension cases match in H20 capacity, does it reduce this dynamic clearance abstract?
Not if you FL size cases a bunch. Here, just forget any measure & matching, as you lose it with the sizing and spring backs and trimming.
If you neck size only, it matters. Your load will be different and you can shoot & see the difference.
Low clearances reduces variance, but amplifies the affect of any.
Then on the flipside, high clearances are forgiving.

I run tight to fitted clearances on everything, so I match H20 capacities after fully fire forming.
I had measured new brass thickness, while checking it's thickness variance.
Before fire forming I had dip annealed ~3/4 down case bodies.
But ultimately, if a case throws a shot, I throw the case.
I don't care about it's brand, cost, weight, capacity, or all the prep efforts. It acted differently.
What makes it act differently is just not what you could predict.
@Mikecr :
Very interesting points you make.

A question for you on variability in shoulder bumps: let’s say we are FL-sizing, and the base-to-shoulder measurements on a comparator vary by up to 0.002”. Are you suggesting that if the shoulders were all bumped back more (eg range is 0.002-0.004”), then we should see less impact on shot-to-shot variations than if the shoulders ranged from 0.001-0.003”, or even 0.000”-0.002”?
 
Last edited:

entoptics

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Jan 16, 2018
Messages
618
FWIW, here's an experiment I did awhile back.


To summarize.

1) There was a reasonable correlation of weight and case volume.
2) There was no measurable correlation between weight/volume and velocity.
3) Virgin brass has lower volume than once fired, but also lower velocity.
 

new2mud

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FWIW, here's an experiment I did awhile back.


To summarize.

1) There was a reasonable correlation of weight and case volume.
2) There was no measurable correlation between weight/volume and velocity.
3) Virgin brass has lower volume than once fired, but also lower velocity.
Great findings!

#3 is an interesting one. I wonder if it correlates to lower neck tension from virgin brass?
 
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Mikecr

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Are you suggesting that if the shoulders were all bumped back more (eg range is 0.002-0.004”), then we should see less impact on shot-to-shot variations than if the shoulders ranged from 0.001-0.003”, or even 0.000”-0.002”?
As long as you're consistent to what you load developed with you're good.
I begin powder development with cases fire formed 3 times. By then the headspace is ~1thou.
After 6 to 8 firings headspace for 30-35deg shoulders will creep to zero & at that point I'll forever bump shoulders 1thou.

[Virgin brass has lower volume than once fired, but also lower velocity] This is true and due to the case expansion energy absorbing I mentioned.
When you take all observations together, and assemble something passing all tests (as truths do), what we seek here is consistent dynamic expansion. Whether weight, capacity, case design, chamber clearances, brass elasticity, it all leads to expansion energies.
 
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QuietTexan

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Texas
Great findings!

#3 is an interesting one. I wonder if it correlates to lower neck tension from virgin brass?
Probably more to do with a higher portion of energy in the shot going to forming the full case body against the chamber. Especially if you aren't FL sizing going forward, the initial shot affects more brass than subsequent shots, especially if you don't FL size to a minimum spec going forward because most of the case is not sized down before subsequent shots.

My thought process (with kind-of guessed at numbers): If the difference in the case and chamber is .0015", the case body moves .0015" radially on the initial shot. Let's say it springs back a full ".0010, it's .0005 larger than before, and then it then only expands 0.0010" on subsequent shots. The net difference of not having to move the extra five ten-thousandths of an inch applies to a SAAMI .308 case diameter of .4539" at the shoulder is 34% less volume increase on the second shot because the cause was already expanded by .0005". So more energy goes out because less is absorbed by the case body. Obviously not perfect because the case is tapered and I pulled the radial clearance out of thin air, but it's my poor redneck attempt at describing exponential growth in the area of circles - relatively small increases in radius result in higher percentage increase in area because area grows at the square of the increase in radius. (engineers - don't cry at my poor math words)
 
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new2mud

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Probably more to do with a higher portion of energy in the shot going to forming the full case body against the chamber. Especially if you aren't FL sizing going forward, the initial shot affects more brass than subsequent shots, especially if you don't FL size to a minimum spec going forward because most of the case is not sized down before subsequent shots.

My thought process (with kind-of guessed at numbers): If the difference in the case and chamber is .0015", the case body moves .0015" radially on the initial shot. Let's say it springs back a full ".0010, it's .0005 larger than before, and then it then only expands 0.0010" on subsequent shots. The net difference of not having to move the extra five ten-thousandths of an inch applies to a SAAMI .308 case diameter of .4539" at the shoulder is 34% less volume increase on the second shot because the cause was already expanded by .0005". So more energy goes out because less is absorbed by the case body. Obviously not perfect because the case is tapered and I pulled the radial clearance out of thin air, but it's my poor redneck attempt at describing exponential growth in the area of circles - relatively small increases in radius result in higher percentage increase in area because area grows at the square of the increase in radius. (engineers - don't cry at my poor math words)
Makes sense.

Here’s an interesting wrench to throw into this discussion :
I’ve got some virgin brass that were sold as seconds due to being drawn to max shoulder length. The ones I have shot have indeed been about 10 FPS slower than the same load in the same brass after it has been FL-sized and shoulder bumped about 0.020-0.030”. So in this instance, the virgin brass is indeed slower than FL-dosed brass, but not because of reduced case capacity or energy being absorbed by shoulder growth, since the virgin brass is LONGER to the shoulder than the FL-sized brass.

Thoughts?
 

QuietTexan

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Radial growth down the length of the case body would have more impact on volume than the shoulder alone. There is a lot more case from the web to the shoulder than just the shoulder that moves forward. The shoulder of a .308 is 0.152" long, the rest of the case body is 1.4082" from the head to the shoulder, that's almost 10x more length, and the shoulder is a truncated cone so it's 30-40% less volume than a comparable cylinder anyways. Your load is 10FPS faster in the second-fired case because the body growth added 10-15x more volume from a .0005" radial increase that didn't have to be expanded on the second shot compared to the lost volume from shoulder having been forward an extra .0010" or whatever your measurement was for the initial shot and being set back some by the die.

I'm trying to say (poorly probably) that I think there is a whole lot more volume in the cylindrical case body at play here than in the truncated cone of the shoulder of the case, so a smaller increase in the case body radius is more impactful than the shoulder moving, even if in absolute terms the shoulder is moving 2-3x further forward than the case body is outward.

Bryan Litz would probably be crying from laughing so hard if he read me trying to explain my thoughts on this... it'd be like Bear Grylls watching a Cro-Magnon make a fire by beating rocks together. Hopefully endearing and not pitiful.
 
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Mike Matteson

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Here several months ago there was a lot of talk on case weight or volume weight. I wasn't aware of volume weight until then. It really lend towards volume weight as a proven way to go. I haven't been able to put much of these application into practices yet (moving). Looking forward to doing those items. I am acquiring better brass. I cleaned flash holes, primer pockets, case weight, and cut necks a very long time ago. I've been planning to volume weight the cases, but now that seem to be going by the wayside do to the input being said here and now. I put groups of brass withing a grain. With all that I was able to achieve about 1/2 MOA at 100yrs, an less than 3" at 500yds. ES was about 30fps variances That was about my limit of yardage at that time in the field and no range finders either. On thing is shooing at the range and shooting animal in the field. Known distances and unknown distance in the field and possible wind to boot.
Bottom line is I will get to go both way to see how it comes out is in accuracies.
Thanks for the input, and got me rethinking this again. Got time so no big deal.
 

new2mud

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Radial growth down the length of the case body would have more impact on volume than the shoulder alone. There is a lot more case from the web to the shoulder than just the shoulder that moves forward. The shoulder of a .308 is 0.152" long, the rest of the case body is 1.4082" from the head to the shoulder, that's almost 10x more length, and the shoulder is a truncated cone so it's 30-40% less volume than a comparable cylinder anyways. Your load is 10FPS faster in the second-fired case because the body growth added 10-15x more volume from a .0005" radial increase that didn't have to be expanded on the second shot compared to the lost volume from shoulder having been forward an extra .0010" or whatever your measurement was for the initial shot and being set back some by the die.

I'm trying to say (poorly probably) that I think there is a whole lot more volume in the cylindrical case body at play here than in the truncated cone of the shoulder of the case, so a smaller increase in the case body radius is more impactful than the shoulder moving, even if in absolute terms the shoulder is moving 2-3x further forward than the case body is outward.

Bryan Litz would probably be crying from laughing so hard if he read me trying to explain my thoughts on this... it'd be like Bear Grylls watching a Cro-Magnon make a fire by beating rocks together. Hopefully endearing and not pitiful.
Yes! Totally makes sense about the cylindrical volume growth from diameter expansion along the entire length of the of the case body.

I should measure case body diameters to see how much they have grown after first firing.
 

Mikecr

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Each system (brass alloy, cartridge design, chamber clearances, load) will exhibit it's own character.
Lapua 6BR tight -vs- Norma 30-06 loose,, the numbers are all different. And these are different than your STW/WSM/WM volumes and designs.

What makes the Ackley improved designs so much easier to manage here is the use of lower body taper/higher shoulder angles and shorter cases per capacities. They need less sizing, so the cases stay more consistent in hardness and springbacks, and volumes.
In the long run they truly are improvements.

FL sizing serves to reduce dimensions, easier chambering, but it cannot undo the changes occurring to brass character.
Once you've yielded brass, it wants to go back to that new balance, It will go back there with less pressure. So people running high pressure loads end up with popping extraction even while FL sizing. They end up polishing out the breach or going ring die for webs.
The root cause of all this is high chamber clearances to begin.

With tighter clearances the brass changes less at given pressure levels. It can't change more because the chamber won't allow it.
That actually means less issue with extraction, because the brass doesn't go so deep into yielding where it would spring back less from chamber walls(potentially leaving interference fit). This is also why it's important to have enough barrel steel around the chamber, and strongest breech support possible(so the chamber does not expand excessively).
Tighter clearances are not a problem load-wise, as you develop loads with it.
That's why it's important to fire form cases to stable(per your reloading plan) -before load development.

Take new brass to it's new form & character, and THEN work with that. Go ahead and plan for it right up front.
 

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