The capacity of that case, after allowing for water temperature, came to 29.86 grains water overflow capacity at 39°F, where the capacity in cc's is figured for European units. If I reduce the length by neck trimming from 1.761" to 1.750" standard trim-to length, the capacity reduces to 29.75 grains water overflow capacity. I rounded that up to 29.8 grains water and added it into the data on the
6mmBR.com page, for which cases were also trimmed to 1.750" length.
I then ran the usual stats and got:
The last number shows that the correlation between between brass weight variance and water overflow capacity variance is only 75%, meaning the brass weight change needed to change water weight is only 6.51 to 1, when the density ratio would be 8.53:1 for 70:30 brass. If the match were perfect, the last number would be 100%. Well, we already know this isn't all 70:30 brass, so a perfect 100% isn't in the cards, but the density difference alone doesn't account for how far off it is. Muntz metal (62:38 brass) density is 8.39 gm/cc. 80:20 (low brass) density is 8.66 gm/cc. Nothing about either gets us down to 6.51 gm/cc, which is where it would have to be for constant outside dimensioned brass to show only a 75% proportionality. Also, I didn't adjust the 6mmBR numbers for likely water density error due to temperature (I should go back and do that, but this is going to be under a tenth of a grain of water capacity, so it won't change things much). Anyway, this all shows the low 75% proportionality is mainly caused by case dimensional differences.
That said, if you look at a scatter plot of case weight v. capacity, you see all the data points lie fairly close to the trendline except the two very heavy ones, the PMP and the old Lapua cases from the 6mmBR.com data. One has more capacity than expected if the external dimensions were identical and the other has less. This has to be due to how wide or narrow the heads were made and how shallow the or steep the extractor groove angle was.
So, I reran the numbers with those two heavy cases left out of the data. That resulted in better average behavior, with the ratio of the weight SD's to capacity SD's just over the ideal 100% and, at 9.01, close to equaling the difference in 70:30 brass density and water density (8.53). The fact the number is a little high is partly random dimensional difference and partly that more case brands are made from low brass than from Muntz metal, so the average density is a little higher than the 8.53 gm/cc of 70:30 brass.
Plotting these:
The 95% confidence limits were ±0.309 grains water overflow capacity. What this means in this case is that predictions of case water overflow capacity based on weight using the least squares line formula will be within ±0.309 grains 95% of the time, or 19 times out of 20. That's not too bad. If I include the two very heavy cases, that number jumps to ±0.75 grains, as those two disturb the confidence level.
The formula for the least squares fit in that last plot was:
Water Overflow Capacity = –0.0872 × (Brass Weight in grains) + 38.48 grains
Try it out on other .223 Remington or 5.56 NATO cases you may have and see how close it comes.
What I think this all means is that most manufacturers are, today, observing pretty consistent and similar external dimensions, new Lapua brass included. Old Lapua can be discerned by its high weight. PMP, I don't know about in terms of old or new, but, again, weight will tell you. The Fiocchi brass fell right in line with the other modern brass based on its weight. With that established, you can follow the old rule of 0.06 grains change in powder charge for each 1.00 grain change in brass weight to keep constant peak pressure. With Fiocchi brass about 5 grains heavier than average, you could reduce charges about 0.3 grains with it to keep constant peak pressure. From many measures that falls withing dispensing precision limits.
