weighing brass

[bruce moulds]

yesterday i weighed 200 lapua 6.5/284 brass.
average wt was 196 gn and extreme spread was 1.4 gn.
this gave 4 batches of 50, 3 with an extreme spread of 0.4 gn, and one of 0.5 gn.
probably not worth weighing, but you have to weigh it to find that out.
let's hope their base hardness (ability to take pressure) is as good as their consistency of weight.
bruce moulds.

[jcinsa]

Bruce,

I had a similar experiance with 300 Lapua 308 Cases, virtually no variation.

Excellent quality Brass.

JC

[Gadget]

Hi,
I just finished 300 x .243's Lapua Blue box. for 2.1g ES starting @172.4g to 174.5g
and being as I could I batched to 1/10th of a grain, (yeah!, I was bored, it was hot, and the A/C was nice) so I finished with 21 sets none less than 13 off and the biggest set had 32off.

Gadget

[johnk]

Now what happens when you fire form them, final trim to length & the weight changes?

[Gadget]

Yep that allways puzeled me as well. but just because I'm a curious sole I weighed a set of 5 times fired, loaded cases, the ES was 3/10 th g.
Now I only did this once and I won't do it again because I don't want to spoil the illusion.

I put a fair amount of time into loading and I realy get upset when I finish and have a couple of round's I think are not prefect,(Nk tension or seat depth off a bit) so I set them for sighters and they hit the 10 ring, you finish your round with 2 or 3 ( perfect rounds in your box) and think to yourself why do I bother.

One day I had a round with almost no Nk tension at all, I took it to the range with the pill in upside down, I turned it over before chambering soft seating it, 1 st shot 2nd detail 10, Bugger me why bother.

Then you have the day that nothing works, Bugger me.

I guess if it was pointing not shooting we wouldnt be doing it.

Gadget

[Woody_rod]

Bruce, I think the importance of weighing brass is inversely proportional to their volume. For example, we spend a lot of time weighing brass for our 223 cases, but not much time on 308's. We have seen a big improvement in elevation for the 223, but no improvement for the 308 doing the same thing.

The 223 being about half the internal volume of the 308, makes weight differences doubly important, due to the percentage of volume the difference of weight applies to.

[Barry Davies]

How do you know that the weight difference represents a difference in internal volume?

Barry

[M12LRPV]

How do you know that the weight difference represents a difference in internal volume?

Barry

You don't because you don't know where the variation manifests itself in the case.

That is why the only way to really do it is to fill each case with water and weigh how much water you can get into it.

Personally I couldn't be stuffed. Maybe one day though, when I can figure out an easy way to plug the primer pockets

[Barry Davies]

Put a spent primer in arse around, seals the hole nicely.
Agree water test is the only way to go, providing always of course that there are no air bubbles in the case-- agree also it's not worth the effort.

Barry

[Woody_rod]

Ok, this needs a proper scientific reply. The idea that material placement makes a difference to the volume is a range tale. I will try to explain it here:

Any given chamber, say a Wylde for the 223 is for our purposes here, a FIXED volume. This is the area swept by the bolt face, chamber walls, up to and including the neck area. The neck, being the most internally and externally accurate part of the case (therefore the most consistent in terms of weight disparity), can mostly be disregarded from a volume point of view. Without going into minute molecular detail the chamber is not elastic, neither is the bolt face. In any case, given a similar pressure for each case, any movement would be similar for each firing, therefore negating any likely small increase in volume.

Therefore:

A case is fired in the chamber, which is a set volume. Assuming that the cases are generally the same outside dimension (which in 99.99% of cases, will be), and the same brass alloy, the weight of the case represents the volume of the chamber that the space the case takes up. The weight of the case and its volume are exactly proportional, with the brass being essentially of the same density.

Excluding the neck itself, no matter where the material of the case is located, it will always take up the same volume. If the case took up 1cc of volume in a case form, a piece of brass the same DENSITY and MASS within the chamber will take up the same VOLUME, even if it did not touch the chamber at any place.

[M12LRPV]

Variations present in the head of the case, particularly around the extractor groove and rim will not contribute cleanly to your theory.

But your correct for the main part. If there is a weight variation it is reasonable to expect that most of it is in the areas that contribute to case volume.

Ultimately it comes down to the tolerances that the individual is prepared to accept and where they are prepared to accept them.


I was initially critical of an individual who weight culled cases before prepping them until I tried this experiment. Take a new case and weigh it then do every bit of prep work you can think of and weigh it again and you'll discover that there is very little variation between the two. It gives a good perspective on just where that weight variation is when comparing case to case. Weigh first, then prep the ones that you are going to keep and sell the ones you aren't.

[Woody_rod]

Variations present in the head of the case, particularly around the extractor groove and rim will not contribute cleanly to your theory.

Actually, that's a good point. I was making some assumptions about the external dimensions of the case. I wonder how much variation there would be, as this is the only real place that could provide variation in the case mass. My guess is that from batch to batch, the accuracy is likely extremely good on Lapua brass for example.

Anyone ever spent time measuring the rim and groove thicknesses?

Your point about culling cases is even more to the point. I have done a bit of the "before and after" weighing myself, and found only very small variations in mass. Which means I suppose, is that it is better to get them close to start with, as you say, than do all that work, then cull them.

[M12LRPV]

Anyone ever spent time measuring the rim and groove thicknesses?

I have encountered significant variations in the groove of lapua 308 brass which I only noticed because of a reasonably close fitting shell holder. As for the dimensions or it's contribution to a weight variation... I never checked because it was not going to be used for an accuracy application.

[johnk]

I recall brass where the angled top of the extractor cutout was higher up the case on one side - can't recall if it was ADI stuff or Winchester.

I would imagine that in an example like that you mightn't get a linear correlation between mass & case capacity.

[Woody_rod]

Anyone ever spent time measuring the rim and groove thicknesses?

I have encountered significant variations in the groove of lapua 308 brass which I only noticed because of a reasonably close fitting shell holder. As for the dimensions or it's contribution to a weight variation... I never checked because it was not going to be used for an accuracy application.

I have also seen this with the shellholder....so it could be a few thou difference? Make that 0,1mm diameter for ISO application (0.004"). (to relate these to inches, just mulitply whole mm by 0.03937 to give thou's)

Circumference = diameter x Pi (10.31mm x 3.142) = 32.2mm (close enough)

Width of groove = 1.73mm

Width x circumference = 55.7mm^2 in area

If the thickness is say 0,1mm @ 55.7mm^2, this gives 0.00557 cubic centimeters (cc)

Brass has a density of approximately 8.6 grams per cc giving around 0.047902 grams for this example.

1 gram = 15.4323584 grain

Therefore: 0.739 grains - less than one grain.

So each 0.1mm diameter difference on the bottom groove diameter (it is actually a bit more) is at least 3/4 of a grain.

This does not account for the angled part of the groove, this will add some weight for each diameter change.