I’ve done many experiments with case neck thickness variations and this is what I’ve found.
I’ve sorted brass within .004-.005” thickness to brass, outside to way outside that variation. This netted a noticeable reduction in uncalled fliers, when all other factors remained consistent.
Things I’ve noted.
Typically, high variance in case neck thickness correlates to variation in case body thickness as well. This is most noticeable in long cases, such as .30-06. Of course, one cannot actually measure below the neck. I conclude this because after firing, the entire case is bent like a banana. I’ve measured the neck in the shorter arc area, and noted this is the thickest part of the entire case. These noticeable “banana” cases are culled immediately.
My theory is that the thicker and uneven neck area does not release the bullet consistently on firing, thus causes fliers. Also the thicker and uneven brass thickness would cause in consistent volume inside the case, causing inconsistent pressure, thus higher ES/SD numbers.
Lapua brass typically has much less variation in thickness.
I’ve done Winchester brass, had to cull almost half of the cases. This brass is much cheaper, and after the cull, the rounds produced very good consistency and accuracy comparable to Lapua cases. Winchester brass usually doesn’t last as long as Lapua cases though. However, even after the cull, one could get much more cases per dollar.
I also have always used the Lee collet neck sizer, after using a Redding body die. Reason being, the neck is sized exactly the same size as the mandrel (on the INSIDE). My conclusion is, this two step sizing and the sorting of low variance neck thickness produces consistent neck tension and bullet release.