Since it is a rainy day and the range is closed I want to kick around a new subject...at least for me.
I believe whole heartedly that one of the most important steps in first sorting of new brass is to measure neck thickness variation. To do this I use the sinclair case neck sorter with a dial indicator guage. The less variation the better IMHO.
Until recently I have always just visually inspected the flash holes of new brass to make sure they were in the center. However, when neck sorting some new 300WSM brass to neck down, I tried measuring what I call "flash hole runout" by spinning cases on the neck thickness variation guage, but taking a measurement at the case web using both the neck sorter and the sinclair runout guage in tandem. Cases are completely unsupported by the case runout guage, I am essentially just using it as a dial stand. Not a perfect system, but it is stable and it gives real measurements.
The better brass (Lapua, Norma) gave very low measurements (0.001-0.002) pretty consistently while the Win brass gave readings all over the place (0.005-0.015) on just a small number of pieces (a couple even up to 0.020). What this tells me is that either the flash holes are oblong (not likely since the spindle fits each flash hole almost perfectly, or the the flash holes can have tremendous variation as to their location relative to the center of the primer pocket. Measuring this runout tells you if the flash holes are centered in the primer pocket.
So... I have re-invented the wheel but,
Better brass are more likely to have centered primer pockets (not rocket science).
Using a tool to measure this runout is probably more accurate than visual inspection. I am sorting brass by neck variation anyway, so taking a reading at the case web is not a big stretch. Sorting by flash hole runout would likely only be necessary once on new brass, so there really isn't a lot of extra work.
Has anybody else tried to measure this?
Does being off center make a difference in accuracy?
If you have the same tools and some unprimed brass, let me know what you think. Maybe there is a better way to measure this?
One could certainly argue that this might not make much of a difference in long range accuracy. I will be the first to say that reading the wind et al. will make more of a difference, but... nonetheless, here we go. Constructive criticism only please!