When you sort brass by weight, you mainly just want to eliminate the extreme deviations from the average. It is not such a critical thing, though. Since the outside dimensions of fire-formed cases are theoretically the same, variations in weight translate to variations of the inside dimensions, and therefore capacity. A reduction in capacity creates an increase in pressure.
However, brass is eight times as dense as water, which has a density comparable to gunpowder. That means an 8 grain increase in case weight is a decrease in capacity of only 1 grain of water. I am told that, depending on the size of the case, a 1 grain variation in capacity has an effect on pressure similar to a 0.1 or 0.2 grain difference in powder charge. So, it takes large variations in weight to make any measurable difference in point of impact.
Actually, in most common rifle cartridges the effect of a change of 1 grain of water capacity is closer to the effect of 0.6 grains of powder to keep pressure constant, though it varies with how well the original powder charge fills the case. This is the kind of thing QuickLOAD is great for, since the changes are relative. For example, in taking a .308 cartridge in a 22" rifle with 150 to 180 grain bullets and changing the case from 56 to 57 grains case water capacity (typical of a shift from Lake City to Lapua brass), mid-pressure charges of either 748 or IMR4895, despite their different grain types and densities, both have to be increased almost exactly 0.6 grains to match pressure and barrel time closely. With a powder that doesn't fill the case well because it's a bit fast for the application, IMR4198, only 0.5 grains is required. For a squib load of pistol powder, like 9 grains of Unique, which leaves 70% of the case empty, then only 0.15 grains is needed to compensate for a grain of change in powder capacity.
Some loads are more charge insensitive than others. In particular, IMR rifle powders with coarse grains seem not to be as charge-particular as some finer grain powders. Dan Newberry has a site devoted to this topic that is a good read. He doesn't like loads that open up groups up with less than a 0.6 grain charge spread. That works out equivalent to 8.5 grains of brass weight difference based on 0.6 grains of powder for each grain of case water capacity.
The last batch of Winchester bulk .308 brass I bought ran from 153 to 159 grains case weight. A 6.0 grain spread. That's the equivalent of 0.42 grains of my charge tolerance based on 0.6 grains of powder per grain of case weight. So his .6 grain load spreads are probably actually the equivalent of 1.02 grain extreme spread loaded into identical perfect cases. That means I have to throw charges within 0.6 grains extreme spread to use that brass and still get best accuracy without sorting by weight. That's +/- 0.3 grains. That's easily done with spherical propellants, but challenging for very coarse powders in many conventional measures. The JDS Quick Measure overcomes that problem for the coarse sticks, but even so, I want as much additional charge grace available as possible to allow for temperature change and powder moisture content, and any other factor that is equivalent to a charge weight change that might crop up over time. With the brass eating up 40% of my charge slop allowance (ironically, grace is slop in this instance), I wanted to at least cut the brass variance in half.
What I actually did with the Winchester brass was go one step beyond simple weighing. I first trimmed all the cases to uniform length, so that trimming accuracy, not great from the factory, did not affect the weight. Next, I went to Walmart and bought a 18"x22" type of poster board they carry that has what they call ghost rule on it. Basically, it’s a faint half inch graph paper grid. I drew a pencil line over the first ghost line that was one grid space in from the edge of a long side and declared that side to be the bottom of the page. I then marked the grid under the pencil line from 153 grains to 159 grains plus a little based on my sample weights. I did this in 0.1 grain steps, with 156 grains at the physical middle. I weighed each case, and lined them up over the pencil line above their weights.
The result of this exercise is stacked vertical columns of cases that form a bar graph in a histogram. It showed how many cases of each weight there were. Instead of the bell curve a normal distribution would make, the histogram had four distinct peaks. That told me the brass likely came off four machines with different sets of tools, and was then all thrown together. So, I proceeded to divide the brass into four groups by dividing them between the peaks as nearly as I could eyeball it.
The next sorting I did was neck wall uniformity, and that resulted in finding one of the four groups had more uniform brass than the others. I made that group my long range match group. I set the less uniform cases from the group aside as more likely having come from one of the other tools, but overlapped this group's weight range. Similarly, I searched out more accurate cases from the two adjacent weight groups, then put all the more accurate cases together on the histogram paper by themselves. Sure enough, this came much closer to giving me a bell curve of cases that likely all came off the same tool. In total, this proved to be about 20% of the cases in my bulk purchase that were what I called my good long range match cases.
It seems like a bunch of trouble, but it’s a Saturday’s undertaking and you only have to do it once in the life of the case lot.
I've been copying & pasting your posts (&others) into Word. I'm gonna have to reheat them a bunch of times to eat all that. So you log and keep your lots together for reference.? How many firings are you getting and what kind of accuracy for you efforts? It seems more in depth than I'll ever need. but I asked! I find that this interest keeps dragging my eagerness that way. I'd Rather have(knowledge) & not need than need & not have. thanks! Lb
I have been weighing brass for my varmint rifles for some time. What I found is that most brass falls into two or three weight ranges. Some time back I did some Winchester .243 brass that had two different weights. I sectioned them and discovered that the heavier brass had a much thicker base than the lighter stuff. This prompted me to load two of them(one from each weight batch) with water. I found that the lighter brass held more water (hardly a surprise). Now for the loads that I do this makes little difference. I have found that the heaver brass tends to be a little bit more accurate given an equal charge (fills the case more). I have not found any real difference in case life.
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