Re: Redding Comp seater runout?
"Boomtube, are you talking about inside neck reaming, or outside neck turning or both?"
Something of a quandry isn't it? What to do, what to do...
Mostly, I avoid reaming because it only reduces neck thickness and leaves any non-concentric inside and outside diferences intact.
Turning (usually) makes the inside and outside concentric BUT, if the mandrel isn't really tight, an off-axis center line (internal vs external cylinder dimensions) remains to some degree. What to do? What I do is neck turn it and if that don't fix the runout, I toss that piece of brass OR mark it and keep it for foulers, etc., if it isn't too bad.
For my factory chambers I never turn more than maybe 75% of the neck circumference, mostly just to clean up the worst of the thickness variation and still have as thick a neck as practical.
When I reform cases and end up with really thick necks, such as making .22-250 from .30-06, I turn to just under MY chamber's neck diameter, not just to max book neck diameter. That gives me near perfect necks but limits its use to myself. But, it's MY ammo and I make it FIT MY rifle, it's not made to fit every SAAMI rifle the factories ever made with that cartridge!
Point is, neck turning can HELP make straight necks for any case but it isin't magic so it can't cure them all.
I don't think reaming, alone, helps accuracy at all but it does insure safety from too thick necks.