Re: Redding competition bushing die set
"I measured a fired piece of brass- 0.338" Loaded shell- 0.332...
Does that mean I have 3 thou clearance?"
No. As you observe, springback must be considered. Your chamber neck is typically at least 2 or 3 thou larger than fired case diameter.
Polishing an expander ball is, in my opinion, over rated. A little sizing lube inside the case neck is far more effective at reducing case neck pulling from the factory expander.
That said, MOST resiized neck tilt does come from the expanding step BUT that is mostly from the massive (to the neck's point of view) change from the sized diameter to the expanded diameter. Some folks hope to reduce neck pull and gain more "bullet tension" at the same time by polishing the expander down another thousant. That's really not good.
Nothing is actually accomplished for real bullet tension by expanding them smaller than 2 thousants under bullet diameter. Smaller that that, the plastic limits of case brass means the neck just stretches enough so the effective tension gripping the bullet remains the same as if it were only 2 thou under. Result; no net gain in effective bullet grip but the necks WILL be slightly bent during seating due to a lack of effective support in conventional seating dies. (That's part of why Forster/Redding straight line type seaters are somewhat better.)
I believe the Lee Collet Neck die is the best bet for factory rifles. It makes straight necks AND the neck's inner diameter isn't greatly under-sized so seating can/should be more concentric.