I've been weighting 257 Roberts case weight. The R.P cases that I have weigh from 168.4 to 174.0gn. Nosler cases weigh between 165.8 - 167.4gn.

Is the 5.6gn variance in case weight for R.P cases enough to make a big difference in pressure when loaded?

Can't help. Do you have enough to sort them as follows?:

bottom 5% cull
6%-50% Lot 1
51%-96 Lot 2
top 5% cull

Then each of the cases in lot 1 and 2 would be within about 2 grains of each other. Just shoot lot 1 cases until they are done, and switch to lot 2. An interesting test would be to make loads from either of the sorted lots and then shoot them against cases drawn from the top and bottom 5%. If you find that there is no difference, just throw them all in a box and keep things simple.

I do have several hundred, but I am only working with 50 cases of R.P right now. I'll try loading 5 on the high end, 5 on the bottom, and 5 center. I'll run them over the chronograph and check for speed variance. I could do H2O as well and see if volume is consistent.

It is difficult to know, as the difference in weight could be down to differences in the size of the extractor groove cut or whether they have been trimmed to the same length.

It is a pretty small difference too: if both lots of brass have exactly the same size extractor groove, are fireformed, sized and trimmed the same, 5.6 gn of brass would equate to about 0.65 gn of water, or about a 1% difference in capacity.

FWIW though it is a good practice to keep batches of brass separate, and if you are changing brass, or any other variable, to be wary, especially if loads are on the high side.

HTH

I pretty much follow utah708's sorting process.
It does not take a lot of variation to effect accuracy.
It does little good to sort brass before it has been uniformed.

And for sure do not mix head stamps.

Case weight can be misleading, what you need to be concerned with is case capacity. There is an article on 6mm Br..com about 243 cases and even though the weights varied significantly the case capacity did not. Sorry couldn't find the article again maybe you can.

The alloy and geometry can affect the weight without affecting the volume much. More than a grain difference in capacity and I will load them separately, usually with charges adjusted accordingly. Use MD's 1/4 rule to figure out what the adjustment should be.

That article was seriously flawed, due to assuming measured case capacity of fired brass related to case weight. It does not, due to harder/softer brass not expanding/contracting the same amount--which also also varies with the number of times the case has been fire.d

While brass alloy does vary, it doesn't vary by much from the basic standard of 75% copper/25% zinc..

Flawed or not the capacity is still the critical point of comparison rather than case weight.