Case weight/volume

[RedSkyFarm]

My old thread was getting long, so I decided to start a new one. I have some probably over shot cases (Hornady). They have been trimmed several times. I weighed them and got close to a 3 gr. difference. How much difference is acceptable to keep consistant (1/2-5/8") groups (100 yds)? Also, in your opinions what are some good, consistent weight cases that can be reasonably loaded several times.

 

[jmorris]

There are lots of shooters and guns that won't ever shoot a 1/2" @ 100 yard 5 shot group even if they used the exact same case for all 5 shots.

Sounds like a good experiment though.

 

[jwrowland77]

For my F-Class loads, if my cases are .3gr difference, I use them all together. It's not going to make that big of a difference. Your barrel is going to like a certain range in velocity. I noticed in my F-Class rifle, there's about a 60fps range that gets the bullets in the same group, so I don't stress as much anymore.


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[Stats Shooter]

Like always, it depends

First of all, the number of grains is an absolute number, so if you are sorting by weight, I should be a percentage. For instance, a .338 LM case will weigh almost 10 times what a .223 rem case weighs. A .338 lapua mag case will weigh around 310 grains so +/- 3 grains is 1%. In a .223 remington case, 3 grains is closer to 10%. The next problem with case weight is that the weight could be in the case head thickness, which would have zero impact on volume.

Finally, I have decided weighing brass is a total waste of time. In both .308, and .223 I separated cases. I made up some loads using the same bullets/powder/primer and charge using cases fired the same number of times, but sorting them by weight. I made up dozens of loads of each using the extreme lightest cases and heaviest cases. The POI, the Velocity, etc did not change.

Environmental factors and differences in neck tension have wayyy more impact than the weight of your brass IF the brass has the same headstamp, is from the same lot, and has been fired the same number of times.

 

[Unclenick]

RedSkyFarm,

It doesn't always matter. If you read the article Bugholes from a Bi-pod, the shooter there said he tried weighing .308 win cases and could never detect a difference in actual results, though I believe he did sort by headstamp. The reason is complicated. Brass lots come off mixed tooling. Even premium brass maker Lapua mixes brass that comes off different forming die sets that are being run simultaneously. No two die sets are a perfect match, but they do pretty well on capacity. What seems to vary more are the heads. Well, if one head is a little wider in diameter than another, that affects case weight, but doesn't affect powder capacity at all, and this is a lot of the weight variation you see.

If you start mixing headstamps, that's another matter. Not only can the head designs vary more, but they don't start out designed for the exact same capacity. But this brings in other problem factors like differences in brass alloy that can affect how fast they work-harden from resizing and how hard the neck grips the bullet. So I never mix headstamps just to keep the variables out of it.

Back in the 1960's, Wm. C. Davis, Jr. did some testing that showed .30-06 brass of different makes had capacity variation such that you needed to alter charge weight about 1 grain for every 16 grains difference in brass weight. But it's approximate. I measured case water overflow capacity and weight of some mixed headstamp .308 once and found that weight among those different headstamps predicted case water overflow capacity with an accuracy of ±20% of the predicted difference. So Davis's rule is going to have that same precision limitation.

Because of that last point on precision, I recommend you sort by headstamp at a minimum. If you are using military brass, you can also sort by year until you are satisfied they are all about the same. Even if you should find cases where weight corresponds to difference in internal capacity, it will take about ±1.6 grains brass weight to cause the same change in barrel time That ±0.1 grains of powder does. It's about the same effect changing ambient temperature 3°F has on a non-temperature compensated powder. So if you are in a situation where the temperature changes 10°F during the course of the shooting day, you would have to have brass ±4.8 grains (a 9.6 grain spread) to see it cause the same issue IF the capacity is changing proportionally with the weight. But if you sort by headstamp that doesn't usually seem to happen much.