Another Basic Question - Why weigh and sort brass?

[privateer]

Sorry for the basic questions but I am relatively new to figuring out how to reload for more than just minute of deer...

Why is it critical for brass to be of relatively consistent weight when reloading for accuracy?

Is it purely a measure of the actual case volume (and free space when loaded with posder and bullet) that determines the pressure profile when fired?

I am assuming that given the same head stamped brass, the wall thicknesses are relatively consistent.

Thx

 

[slickoneuk]

yep, if a case has the same dimensions it should have the same weight. As we can measure outside measurements and weight easily that just leaves the inside measurement as a variable. For super accuracy you want to eliminate all unknowns. Same bullet weight, same charge of powder, same seating position, same neck tension, same trigger pull, same stock mount etc etc etc. If you can rule out one more variable by weighing and estimating the internal volume you do it. Just another step towards the consistant load, then if you miss, it your fault! Good shootin

 

[cmillard]

it can be a matter of safety also. i loaded my normal .223 load in weight sorted brass of 91-92 grains. I then tried the same load in cases weighing 94+ and i had primers pop out of the cases. The heavier the brass, the lower the internal volume, therefore more of a spike in pressure.

 

[CJ6]

Actually it makes me FEEL better that I've done all I could to make my loads as close as possible ;D. Then there is only 1 excuse the WIND DID IT :

 

[privateer]

Thanks for the responses.

 

[Elkbane]

It's not a simple as is seems. Just assuming that there is a 1:1 correlation between weight and volume is erroneous.

Example: Ever noticed when you are trimming cases and you put the base of the case into a collet type holder (think Forster), that the handle you turn to tighten up the collet turns tighter on some cases than others? This is an indication that the base has a higher OD. The base is the heaviest part of the case. Imagine how much vairation that alone imparts to a weight segregation.

Now, take your weight sorted cases - on one "like weight" batch of brass, and actually measure the case rim with a micrometer. See any difference? If so, your weight sorting has not accomplished what you are trying to do, which is get consistent case VOLUMES.

If you want to do this, just recognize its limitations.
ELkbane

 

[cmillard]

sorting cases by internal volume is a better idea. fill the cases with some type of liquid and then weigh the case (obviously weigh the case before filling also). sort by the amount of water that was in the case. but, with doing this, you want to make sure that the casings are all trimmed to the same length and the necks are all of the same thickness. use fireformed brass to do this, and leave the primer in so the fluid does no exit the bottom of the case.

 

[Ackman]

If you're wanting to actually see a difference in accuracy, weight sorting brass won't show you anything.

 

[Steve Blair]

Ackman: What evidence can you present that will validate your statement?

A recent study I performed weighing 500 Winchester .223 cases and then measuring post-firing case volume samples from both extremes shows a strong Pearson correlation between weight and internal volume. If you are looking for a one-to-one correspondence; there is not. By weight-sorting, the amount of internal volume variance is reduced, aiding the overall goal of consistent loads.

The two extreme groups printed ~1.3" apart at 300 yards with the same load. The groups were fired from a benchrested heavy-barrel 40XBR. The "light" group weighed 93.70-93.78 grains and the "heavy" group weighed 95.70-95.78 grains. The 2 grains represents a little more than 2% case weight difference. The light group had an average internal volume of 31.0 grains and the heavy group 30.4. That difference is just under 2%. QuickLOAD says the difference is worth 3150 psi for my test load.

The attached JPEG is a histogram of the 500-sample lot.

 

[fivering]

Ackman:

I'm afraid you are over looking some very basic principles here.

If the brass is all trimmed to the same length, and the outside dimensions are the same, either from fire forming or resizing, then it follows the empty volume of the case would be the same for cases that weighed the same, no matter what the distribution of the metal was, as it is a fixed weight per unit of volume of the metal.

There are some fine points as pertain to accuracy, as it is certainly best if the case walls are of uniform thickness, etc, but after uniforming the new brass to length, the best thing you can do is weigh your cases.

 

[ronemus]

I did an experiment several years ago to determine just how much effect brass weight has on .223 loads. I used WW brass (sized, trimmed and deburred, primer pockets uniformed, flash holes deburred, and neck turned) , WSR primers, charges of RL-15 or N-550 powder weighed to 0.1 gr, and 75 gr A-Max bullets. Using the lightest and heaviest cases (sorted from 1000 once-fired I had on hand), I had two lots of 10 cases with a 3 gr difference in weight. The average muzzle velocity difference was 16 fps, just a bit more than the 12 fps due to 0.1 gr of powder. I choose to sort 0.5 gr lots of brass for my long range loads, but the effect will only matter at 800-1000 yards - the vertical displacement on the target from such a small velocity change is negligible at shorter distances. Unless you control all other sources of variation, the effect of brass weight is negligible.

 

[Steve Blair]

The difference will matter more as you approach maximum loads. When pressure is near maximum, small load variations cause a greater pressure difference. Generally, but not always, my long-range loads tend toward maximum pressure as that is where I get best ES/SD. Accordingly, I weigh those charges on an accurate scale and minimize variation as much as possible. One variable is case weight. As Elkbane observed, there is no direct correspondence, but there is a relationship between case weight and case volume. The external case variables of rim diameter, extractor groove, etc., that are not constrained by the chamber, disallow a direct correspondence.

BTW, I also tested my volume measurement process capability on one case, emptying and refilling with water twenty times each with various methods. My best maximum process variance was .1 grain. That introduces another uncertainty into the overall measurement. I tried various methods such as using a credit card to "level" the top, a piece of paper towel to "wick" away excess from the side, etc. My best results came from sighting across the neck with a strong light and gently touching the meniscus edge with a paper towel to achieve a level surface.

 

[memilanuk]

Steve,

I performed a similar test a couple years back (for a final paper for my Stats class ) - 50 pcs of Winchester .308 Win brass, and found a correlation between unfired weight and muzzle velocity. It wasn't especially strong (0.44) but it was enough to qualify as 'statistically' significant.

If you don't mind sharing, I'd like to see your raw data sometime.

Thanks,

Monte

 

[Ackman]

Ackman: What evidence can you present that will validate your statement?

It's about cases having the same internal volume. With fireformed - uniformed - weighed .223 cases, WW and IMI headstamp......caseweight wasn't an indicator of internal volume. I've posted about this several times.

 

[Ackman]

Ackman:

I'm afraid you are over looking some very basic principles here.

If the brass is all trimmed to the same length, and the outside dimensions are the same, either from fire forming or resizing, then it follows the empty volume of the case would be the same for cases that weighed the same, no matter what the distribution of the metal was, as it is a fixed weight per unit of volume of the metal.

There are some fine points as pertain to accuracy, as it is certainly best if the case walls are of uniform thickness, etc, but after uniforming the new brass to length, the best thing you can do is weigh your cases.

No. Weigh some brass shavings....it takes hardly any brass volume to weigh a grain or two or even three. It won't necessarily be in the case body. Just spread out in the neck, that small volume would make little difference in neck thickness. And that volume can be anywhere on the case, including the head and extractor groove. Some cases are tighter in the shellholder than others and I've had .223 cases that actually wouldn't fit into the shellholder.....that's brass volume.

 

[Steve Blair]

[quote author=Ackman] I've posted about this several times.[/quote]

I am not trying to provoke you, but posting does not constitute evidence. I asked whether you have anything to validate what you say. I have run several tests, tried to account for process uncertainty and attempted to prove whether case weight is an accuracy contributor or not. I believe that my data shows a correlation between sample population weight distribution mean and case volume distribution mean. That means that a subset of weighed cases, selected by weight, when fired and volume measured, show a statistically significant bias away from the total population mean toward the sample weight. It does not mean that the lightest case has the largest volume and the heaviest the least. It does mean that, within my tested populations, case volume tends to follow case weight in an inverse relationship.

Monte - I will put the data into better order and send it to you. Send me a PM with an email address. Would you prefer Excel or CSV?

 

[memilanuk]

Steve,

Either one would be fine. Excel if you have it, otherwise I'm reasonably capable of translating between the two

Monte

 

[TonyR]

I conducted a series of tests after weight sorting and preping 500 new Lapua 6.5X284 for my 284 Shehane. I have proven to myself that there is a statistically significant relationship between case weight and velocity but it is imperfect for the reasons others have stated. I tried to see if the correlation would improve if I measured the internal case volume instead but, even using a lab quality burrette, the water weight of a given case was not repeatable. I measured all of these cases before and after trimming and neck turning and cases that weighed the same before turning and trimming did not weight the same after so there must be differences in the way the brass is distributed withing the cases. As a result of all of this, I believe that sorting is worth the effort but that there is opportunity for further improvement by methods I haven't yet discovered.

 

[Steve Blair]

Monte,
Excel it is. I did not know whether you were using SAS, SPSS, R or some other statistics software. I do all my poor analysis in Excel, currently 2007.
Steve

 

[rain164845]

Ackman: What evidence can you present that will validate your statement?

A recent study I performed weighing 500 Winchester .223 cases and then measuring post-firing case volume samples from both extremes shows a strong Pearson correlation between weight and internal volume. If you are looking for a one-to-one correspondence; there is not. By weight-sorting, the amount of internal volume variance is reduced, aiding the overall goal of consistent loads.

The two extreme groups printed ~1.3" apart at 300 yards with the same load. The groups were fired from a benchrested heavy-barrel 40XBR. The "light" group weighed 93.70-93.78 grains and the "heavy" group weighed 95.70-95.78 grains. The 2 grains represents a little more than 2% case weight difference. The light group had an average internal volume of 31.0 grains and the heavy group 30.4. That difference is just under 2%. QuickLOAD says the difference is worth 3150 psi for my test load.

The attached JPEG is a histogram of the 500-sample lot.

What were the group sizes at 300 yards?

Joe