Ysterhout Dot Net
It is possible to measure an amount of runout on a case whose neck axis is precisely aligned to the case body axis. This measured runout is the variation in neck wall thickness as the case is rotated. All brass has a couple thou variation, and this can make you believe you are measuring actual instead of neck thickness variation.
Consider that brass rapidly work hardens, work hardened brass deforms itself, and has springback. You may turn your necks to uniformity, mandrel size, and still encounter runout, in which case your case necks need to be heat treated. During your case preparation operations, and between successive firings, the brass work hardens. As it work hardens, it builds up a non-uniform strain matrix, which may cause the brass to bend a little in one direction. It also tends to resist deformation, so after straightening the necks, they tend to revert towards their previous axis.
The decapping rod assembly with expander ball, or the seating stem, can induce runout.
Every full length sizing die has a decapping rod in it, and on that rod is some form of neck expander.
The expander ball passes through the resized case neck on the downstroke. The longer and thinner the rod holding the expander ball the greater it's possible deviation from center, and the higher the probability it contributes to runout.
Lee Precision and Forster produce a decapping rod assembly with a neck expander that engages the case neck closer to the beginning of the downstroke. This gives the expander ball better alignment to the axis of the die, resulting in less potential measured runout compared to other makes. Lee have changed their design to a long taper machined as part of the decapping rod, instead of a ball, and this is a big improvement over other brands that still use a removeable ball.
When the decapping rod is not perfectly straight, the expander ball passes through the case neck at the proportional angle, and this is a physical factor affecting runout. When the expander ball is higher up on the decapping rod assembly, the proportional angle is smaller.
The seating stem can induce runuot in two scenarios :
- When the seating stem does not make firm contact with the bullet ogive, and bears only on the bullet tip, the bullet can seat at an angle to the case neck axis. This forces the case neck axis out of alignment with the case body axis, inducing runout.
- When the seating stem is too small for the die, and there is excessive play between the seating stem and the die body, the bullet can be seated out of alignment with the case neck axis, again inducing runout.
