Re: Concentricity Gauge
I just turn my case necks only enough to clean them up if there not concentric in regards to thickness. One habit I got into years ago is when I'm turning necks on a new to me caliber is I will take every case I can get my hands on for that caliber and inspect them all, looking for the worst offender (being that case that has the thin side thinner than the rest) and set my neck turner to just barely clean that one up. My reasoning is that from that point all the rest will also be cleaned up and of consistent neck thickness. I will mic my turning tool and record that setting which is valid only for that caliber, so in theory with cases purchased at a later date can then be neck turned to the same dimensions.
After that, the other factors that I can see effecting runout are sizing and seating dies, and just how good your chamber was cut. If your dies induce runout, the only real fix is different dies, though I can see where turning the loaded round in the shell holder and bumping it in the seating die could help. If your rifle is of high manufacture rate you could very well have a chamber that is not quite concentric to the bore, or the neck portion isn't concentric with the case body portion, I can't imagine where a guy could do much at all that would mean anything without a visit to your GS of choice.
Keep in mind the animals we shoot for food and display are not bullet proof. Contrary to popular belief, they bleed and die just like they did a hundred years ago. Being competent with a given rifle is far more important than impressive ballistics and poor shootability. High velocity misses never put a steak in the freezer.