Re: .308 case capacity lapua, hornaday
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 918v</div><div class="ubbcode-body">
This is for fireformed brass, right? Cuz a new Win case holds 56 grains ow water as measured with a convex meniscus. </div></div>
Yes, this is for fireformed brass. Unfortunately, case water volume determination is *ridiculously* technique sensitive. It is difficult to compare absolute volume numbers from one reloader to the next. If I was asking another reloader about case volume for a case that I hadn't measured, it would be more useful to ask them about the differences between "brand-x" and a particular brand we both had in common. The reason I weigh with a flat meniscus is that due to surface tension, an "overflow" convex meniscus can contain slightly more or slightly less water. Not much, but definitely measurable.
My technique is probably more "tweaky" than most would be interested in (my major in college was biochemistry). Basically I set all 10 of the cases I'm measuring in a reloading block and use a pipette to get the liquid levels all slightly convex. At this point, I get a shred of paper towel and literally "wick" the excess H2O off until the mouth of the case is a perfect undistorted mirror. This takes longer than most techniques, but I have 100% confidence in the numbers. As before, they are really only useful for comparing relative volumes.
As an added double-check, I also calculate the standard deviation of the water volumes for a given sample of cases. If the sD is overly large, I will go back and redo the measurements as the discrepancy is either my technique, or a mediocre batch of brass. For the batch of Lapua in the previous post, the sD for water weight was .093 grains.
Darrell
