Weighing cases

What's the big deal about weighing cases?

You are probably asking "Why should I weigh my brass and why should I even care?". Most handloaders have read about sorting cases by weight and some are somewhat confused about it. I mean, what can be simpler than a brass cartridge case? All it does is holds the powder, right? Actually, there is a little more to cases than that. Follow along and we will take a look at what I mean.

First off, let us look at what case sorting by weight is. This is a technique that is used to determine how consistent the brass of the case is. The closer the cases resemble each other the more consistent the results of the load will be. The reason is that brass of the same thickness and same length will give the same results each time if the same powder load, primer, and bullet is used. As we all know, one of the keys to accuracy is consistency.

Following the logic that the cases are made on the same machine and from the same sheet of brass we can determine the consistency of the cases by weighing them. If the case weight is too high or too low, it will likely have a case volume that is inconsistent with the others and produce less than acceptable performance. This is because thicker brass will be heavier, thus creating a smaller internal volume. This smaller volume will create higher pressure if all other aspects of the load are equal. Lighter cases will have a larger internal volume and produce less pressure.

Now that we know what weight sorting is and why it's done we will look at how it's done. The first thing we must do is get a scale that is accurate. Some loaders will use the same balance beam scale that they use to weigh powder. Others use an electronic scale such as those made by Lyman, RCBS, and PACT. I use an electronic labroratory balance that we have at work as it is super accurate and convenient. No matter the equipment used, you must verify it's accuracy with some sort of check weight. Once you are convinced of it's accuracy you can begin.

Obviously, we will need some cases for this. Choose whatever brand you want, the theory works the same. Most loaders will buy at least 100 cases to start with so we will work with that number.

Now that you have the cases, it is time to full length size them. Run them through the dies as you normally would. Once all are sized, clean them thoroughly to remove anything that may add to the weight. Trim the cases to the desired trim length, turn the necks now if you plan on doing it, and debur the flash hole. If you plan on doing anything else that will remove material from the cases, do it now.

Let's start weighing the cases. I like to take a random 25% sample of the cases to weigh for the standard. This is basically the average weight that we will use to compare the cases to. For this exercise, that will be 25 cases. Some loaders will weigh each and every case to produce their standard but those folks are the ones who generally don't have any friends.

We will take the standard that we just established and use to compare all of the cases to. I like to cull any cases that are not within 1% of the standard. For example, if the standard is 150gr, 1% is 1.5gr and I will cull all the cases that are more than 151.5gr and less than 148.5gr. When culling, place all the cases that are within the standard in their own pile, box or bag. Place all the cases that failed in the respective piles, one for lights and one for heavies. If there is a significant amount of either, you then weight sort those using the same method. This will allow these cases to possibly be used if they are kept seperated from the rest and labled.

Here are some examples of weight sorting using the 1% method:

Cartridge- .223 Remington; Brand- Lake City 02; Cases weighed- 25
High- 93.36gr
Low- 91.35gr
Avg- 92.18gr
Standard = Avg X 1% = 0 .92gr, acceptable range: 92.18gr +/- 0.921gr = 91.25gr to 93.10gr.
Found four cases that were too heavy and none that were too light.

Cartridge- .308 Winchester; Brand- Lapua; Cases weighed- 25
High- 173.61gr
Low- 171.19gr
Avg- 172.67gr
Std= 172.67 X 1%= 1.72gr, acceptable range: 170.95gr to 174.39gr
All cases were found to be acceptable.

Cartridge- .338 Winchester Magnum; Brand- Winchester; Cases weighed- 25
High- 236.88gr
Low- 234.15gr
Avg- 235.33gr
Std= 235.33gr X 1%= 2.35gr, acceptable range: 232.98gr to 237.68gr
All cases were found to be acceptable.

The above cases were weighed by me on an AND HR-120 electronic laboratory balance.

Many handloaders like to use the 1gr method of case weight sorting. They basically weigh all the cases and sort them into piles like in the above method. However, the standard that they use is 1 grain instead of 1%. The only problem I found with this method is that the rejection rate for small cases such as the .223 Remington is too small as practically all cases will be within 1 grain. The bigger cases experience a higher rejection rate because as the case gets bigger, the case weight naturally gets larger. Good cases get culled because they can't meet the 1 grain standard. The 1 grain standard is not a bad method by any means though and if you are getting good results from it, then by all means use it.

Jcalhoun, I appreciate your post as many precision shooters weight their cases for the reasons you've elaborated. However, I do not share your views as regards the benefits of this practice.

I believe that weighting cases on the assumption that there is a constant correlation between case weight and internal volume is a complete waste of time that is based on an erroneous assumption. Others factors such as dimension of base and extractor groove are significant confounding variables.

With new brass, I expand necks to uniform size, turn them for uniform thickness, trim to uniform length, chamfer, clean primer pockets and deburr primer holes. After fireforming, I routinely measure internal volume of 30 cases in order to adjust Quickload parameters to customize it my firearm. I have done hundreds of these measurements and I have found that even with the best of brass (Lapua), the correlation between case weight and internal volume is extremely weak and typically less than 50% valid.

Nothing like a little data to illustrate the point. Here's a sample of 30 fireformed Remington cases in my 338 Edge. All cases have been prepped as described above. The empty and water-filled cases were weighted on an Acculab VIC 303 that is accurate within .001 gram or .02 grain. A drop of dishwashing soap was mixed with the water used for the tests to reduce surface tension and ensure consistency from case to case.

#.....Case weight (grs)……Var. %……..Water weight (grs)….Var. %
1…….272.68……………………-1.19%.............116.66…………….-0.04%
2…….276.74……………………+0.28%............117.00………… ….+0 .25%
3…….272.68……………………-1.19%.............117.10…………….+0.34%
4…….267.24……………………-3.16%.............117.48…………….+0.66%
5…….278.84……………………+1.04%............116.18………… ….-0.45%
6…….276.24……………………+0.10%............116.62………… ….-0.07%
7…….278.14……………………+0.79%............116.12………… ….-0.50%
8…….276.22……………………+0.09%............116.56………… ….-0.13%
9…….280.58……………………+1.67%............116.24………… ….-0.40%
10…..276.48……………………+0.19%............116.40………… ….-0.26%
11…..276.58……………………+0.22%............116.50………… ….-0.18%
12…..276.76……………………+0.29%............116.64………… ….-0.06%
13…..279.64……………………+1.33%............116.64………… ….-0.06%
14…..278.16……………………+0.80%............116.80………… …+0 .08%
15…..277.18……………………+0.44%............116.78………… …+0 .06%
16…..277.60……………………+0.59%............116.64………… ….-0.06%
17…..275.82……………………-0.05%.............116.82……………+0.10%
18…..271.74……………………-1.53%.............117.20……………+0.42%
19…..276.40……………………+0.16%............116.72………… …+0 .01%
20…..271.68……………………-1.55%.............116.86……………+0.13%
21…..275.62……………………-0.12%.............116.94……………+0.20%
22…..276.32……………………+0.13%............117.22………… …+0 .44%
23…..276.00……………………+0.01%............116.96………… ... .+0.22%
24…..278.02……………………+0.75%............116.74………… …+0 .03%
25…..273.90……………………-0.75%.............116.54…………….-0.14%
26…..278.00…………………..+0.74%............116.52……… …….-0.16%
27…..275.60……………………-0.13%.............116.10…………….-0.52%
28…..276.62……………………+0.24%............116.74………… …+0 .03%
29…..275.44……………………-0.19%.............116.74……………+0.03%
30…..275.94……………………-0.01%.............116.72……………+0.01%
Ave..275.96………………………..…………………....116.71

Using the 0.5% variation standard normally advocated (+ or - 0.25% from average), one would have selected 13 of the 30 weighted cases. Selecting by internal volume would have resulted in 20 retained cases. Moreover, 3 of the cases selected by weight would have been rejected by volume (#10, 22 and 27). Consequently, selecting by weight would have resulted in the identification of 10 of the 20 cases that had relatively uniform internal volume.

If one looks at the validity of the assumption of case weight as an indicator of internal volume within a 0.5% variation, it would appear that the assumption is dubious at best. For example, you would expect case #1 which weights 1.19% less than the average of 30 cases to have approximately 1.2% greater internal volume than the average volume for the same cases. Instead, that case is almost perfectly on the mark for case volume (-0.04%). Indeed the correlation is valid only for 11 of 30 cases (#6, 8, 10-12, 17, 19, 21, 23, 29 and 30) or 36.7%.

Another interesting observation from the table above, is that the weight variation among cases is quite large (-3.16% to + 1.67% = 4.83%), whereas the internal volume is relatively constant (+0.66% to -0.52% = 1.18%).

I and 2 of my shooting buddies have run this same experiment independently with hundreds of cases, using 5 different brands of brass (WW, RP nickel, RP, Hor., Lapua and FC) and 6 calibers (223 Rem, 6.5x284, 308 Win. 300 WM, 338 WM,338 Edge) and have come to the same conclusion. That is, if you wish to sort brass for consistency, do it by measuring internal volume.

Moosetracker, I would suggest that the water method is only valid if all the cases have been fired so that they conform to your chamber, AND they have all been trimmed to the exact same length. Without those 2 qualifying factors, the internal volume method is meaningless. Thus, for new brass, where the exact geometry may not be repeatable, weight is a more reliable method for sorting. For once fired brass (in the same chamber) with primer in place and the length all trimmed to the same value, the water volume method may be more accurate. I suppose since I can neither afford, nor locate new brass currently, using the weight of the shell has worked well enough for me. I don't do anything more competitive than kill coyotes.

Quote:

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Originally Posted by Westcliffe01

Moosetracker, I would suggest that the water method is only valid if all the cases have been fired so that they conform to your chamber, AND they have all been trimmed to the exact same length......

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If you read the 3rd paragraph of my post, you'll see that's exactly what I do!

Sorry, I missed that part....

What this suggests is that the base of the cartridge has a fair amount of variation. When one considers that cartridges are made by impact extrusion, the outer geometry (prior to necking) is defined by an outer die and the inner geometry is defined by the extruder die. If the springback of the brass slug varies then the base of the cartridge may be thicker or thinner. The wall would tend to be more consistent and any variation in overall length is taken care of when trimming. Volume of a cylindrical body is proportional to the square of the diameter but only directly related to the length of the cylinder. Brass is about 8.5x as dense as water, thus a little extra brass might change the weight a lot, but actually change the internal water volume little.

When I was sorting through a recent batch of 500 "Fiochi" 308 shells, I found about 30 cases that were 5 grain light and about 50 or so (10%) that were 7-8 grain heavy. It turned out that all the heavy cartridges were either S&B or PMC (different head stamp) whereas the light ones were from the same manufacturer. 5 grain light (brass weight) translates into 0.6gr water weight which is considerably less in powder weight. I have kept this brass in the 3 categories, because I would not like to randomly have -5 to +8 grain variation in my brass weight but neither of the outlier groups is big enough to justify a batch on its own. For now it just goes into the "reserve" pile...

Good stuff. Luckus

The water volume method would work well on fired cases. I should have stated that I was using unfired brass.

Anyway, that was an article I wrote several years ago and recently found it again, without the pics of course.

The main point of the article is that I prefer to use 1% as a standard rather than the more common 1 grain method because of the weight differences between small cases and larger cases.

Also, I don't weigh cases unless it is for when I'm shooting a competition.