Acceptable variation in case weight?

[ronemus]

One way to approach this question with a minimum number of loaded/fired rounds is simply to choose about 5 cases from each end of the weight spectrum (i.e. both extreme low and extreme high weights from within the total range). Load them identically in terms of charge weight, seating depth, primer, etc., then determine an average velocity for the "light" and "heavy" groups for direct comparison.

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 .223 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.

I also shoot .284, and because the brass is twice as heavy I batch in 1 gr lots.

I shoot .338 Lapua Magnum with brass weighing what a .223 does, but I have too few cases to sort into reasonably sized lots. I sort them in sequential order instead - a 10- or 20-shot string will have a spread of <1 gr, so elevation won't be affected significantly.

 

[SmokinJoe]

I think the old standard was 1.5% spread on the batch. BR shooters, which includes me, can be pretty anal about this stuff sometimes. When I was competing with the 30BR years ago, I actually bout 2000 pcs (2 cases) of Lapua 6BR brass and seperated it by weight. Ended up with 1200 that were within 0.5 grains. Still have 350 pcs left. Going to make some 6 Dasher out of some of it with a Whidden's hydro form die that I got today.

 

[Doom]

One grain of brass weight is about 0.11 gr of powder capacity in a 223 capacity cartridge (~0.4%), 0.10 in a 308 (~0.2%) and even less in a magnum sized cartridge. This assumes all the weight variations affect capacity and are not in the case head. Maybe someone with Quickload could run some comparisons for this.

 

[Rocketvapor]

I agree that powder capacity is changed by about 1:8 or 1:9 of the brass weight difference.
But that is empty space that is changed, not charge.

External dimension of the rim or groove might cause weight variation but if it is in the web thickness, side wall thickness near the base it will impact volume. Look at a cross section of a case and add/remove brass that does not change volume. A thicker web adds weight but also reduces volume.
Incomplete list

Brass Counts by Weight - Arm or Ally

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93 grains.............................................................................................................. 111 grains


126 grains, 6mm HAGAR sized, bumped, trimmed, for 22NOSGAR. Not a real clean cut


Capacity difference between 22N and 22Nosgar, weight difference 15 grains. Reversed spent primer in these two. Water volume would be better but powder capacity is easily seen. Some of the weight difference is in the rim.


Load reduced about 0.5 grains in the Nosgar closely matches the velocity of the 22N.

 

[Rocketvapor]

Sitting around with nothing to do
Just for the fun of it, grabbed some .30-06 mil cases and found a range of 198 to 202gr (primed).
Took the heavy one and removed the rim until it matched the weight of the light one.
How much of the variation was in the solid base/rim/groove?
Maybe I'll check capacity with water.
?

 

[moorepower]

What may be substantial weight variation in a .17H would not be noticeable in a .300 WM. I sorted 300 pieces of Federal .222 and loaded up the far outliers and shot at 100yds and could hardly tell them from those with identical weight. For Pdogs, out to as far as I shoot the .222, I can't say i would ever miss do to the odd weight brass.

 

[Rocketvapor]

Even with the .30-06, +/- 2 grains in case weight probably would be hard to see.
Except @ 1000 yards or so.
I hope I have made the point that MOST weight variation is NOT the rim/groove and will likely be seen in volume.

 

[243winxb]

The rifles chamber has a volume. Runs from bolt face to where the bullet seals the bore.
Any change in mass, placed in the chamber volume, changes pressure on firing.

 

[Rocketvapor]

While this sounds intuitively correct, it probably applies to a single chamber.
Put more brass in the chamber, less internal volume for powder, higher load density, higher pressure.
If this worked across various chambers and different calibers it would negate most internal ballistic calculations.
Brass, steel and aluminum cases can have different mass for the same internal volume.
Some even question variations in brass density. Or blame rim thickness for case weight variations.

An interesting situation is cheap X39 steel case ammo.
Steel case weight variation of 5 to 6 grains and bimetal projectile vairation of 3 to 4 grains.
A large sample of the completed cartridges includes independent processes to make the case, the bullets, charge and seat. These are distributed through the final product.
Heavy case/light projectile, and light case/heavy projectile can have the same mass, but will perform differently.

 

[ronemus]

As pressure builds, the case wall is pressed to the chamber wall well down the case, probably to just above the case head, and the head is driven to the bolt face; I consider that the relevant volume of the case, not the volume after spring-back. The void in the extractor groove and between the case head and chamber wall are the only voids when the cartridge is fired.

 

[Rocketvapor]

I agree but,
What is the expanded volume compared to spring back volume of a .223? .30-06?
Which do you use for loading?
I guess you could measure case volume after sizing, and after 1, 2, 3 firing without sizing to get an approximation of volume under pressure.

Drifting off track, and it's only page two
While not perfect, weight sorting takes care of brass mass inside the chamber and final expanded volume.

 

[ronemus]

That would get closer, but would not account for the spring-back, especially if it was not a tight neck.

 

[Rocketvapor]

Not disagreeing, but what is the volume difference?
I would think you have to know for it to be beneficial data.
I guess you could calculate a volume with an assumed 0.001" cylindrical diameter spring back, and a 0.002" length growth. Is this in the order of a few percent change in volume? or 0.1%?

Rough guess using cylinders;
1/2" diameter cylinder, 2 inches long has a volume of 0.3925 cu.in.
A 0.001" larger diameter cylinder, 2" long has a volume of 0.39407 cu.in.
(0.4% percent larger plus 0.1% for 0.002" length growth).
For a 200 grain ..30-06 case 1/2% is equivalent to a whole grain of brass.

 

[ronemus]

That depends on the size of the case. The temperature dependence of the size of brass can be obtained from a handbook, but it might depend on the state of annealing. The head and most of the wall are probably fully hard, but the neck and shoulder would have been annealed (not fully) at the factory and would harden with each firing. Knowing the temperature of brass immediately after firing (probably caliber-dependent), the spring-back could be calculated.

Rather than go to such lengths, I just weight sort. As has been demonstrated above, the extractor groove would not vary much, so weight provides a useful indication of what the relative volume under pressure would be, Personally, I'm not concerned with calculating internal ballistics (too many approximations unless a lot of experimental measurements are taken); easier to make a few trials at the range.

 

[Rocketvapor]

Weight sorting sounds good to me.
To account for large/small cases, heavy/light projectiles, charge weights,
a percent based variance is a easy metric.

 

[243winxb]

https://forum.accurateshooter.com/threads/effects-of-case-weight-variation-on-accuracy.3972946/

More on subject.

 

[Rocketvapor]

I linked that thread earlier.
I suggest anyone that thinks the groove, rim, primer pocket, has more than a very minor effect on case weight, do some trimming to see how much it takes to remove 1/2% of the total weight.
I took the entire rim off a .30-06 case in a previous post. How much weight was that?
Weight in the "case head" above the primer pocket of course adds weight but it is in the volume area of the case. Web thickness adds weight but reduces the powder column height (volume).
Weight a case, uniform the pocket, weigh the case. Clean up the flash hole weighing before and after.
Shoot it 3 times and trim to length weighing the material removed.
These are VERY small quantities.

One issue with water weight measurements is the precision needed with something that is 1/8 the density of the brass.
Calibrating glass labware to better than 1/2% is difficult, even in a controlled lab environment.

If you want to measure cases by volume you must have:
Constant temperature. Temperature of first case to the last case.
A wetting agent in a small quantity that will not change the density of the water.
The inside surface of the brass must be clean and smooth.
A FLAT meniscus, neither concave or convex. A meniscus is useful in reading increments from one graduation to another. Notice how a single marked flask is made. You view the bottom of a concave meniscus through the flask aligning the mark on both sides of the neck.
You need a measurement accuracy (weight) better than the results expected.

Grains to 2 decimal places would be just adequate. Sort brass in 0.1 grain increments, be able to measure water weight to 0.012 grains.
Sort brass in 0.25 grain increments, measure water weight to 0.03 grains.

Practice on Class A volumetric glassware. About +/- 0.2%
Select one or more sizes for your cases.


When you master that, try cartridge brass.


Wicking to a flat meniscus



30 cal neck, about 0.6 grain difference between case on the left and on the right.
Practice with a single case. Add a drop for a positive meniscus, wick down to flat. Repeat and determine your repeatability. If you can't repeat to less than a tenth of a grain, you be doing something wrong.