Re: Seating Consistency w/ CO-AX and Redding Comp. Die
There are a few reasons we should always expand before seating bullets, and as mentioned an expander mandrel is best for this(Sinclair's expander die sys).
-Pushes thickness variance in necks outward for straighter seating/lower loaded runout.
-It injects energy into brass that will try to release over time(back to balance). This release is called springback,, it's all that is gripping bullets,, and it continues over time. It's conservative to leave springback strengthening bullet grip over time(well, trying to), rather than weakening(much easier). When you downsize necks as the most significant action(rather than expanding last), springback is outward away from bullet grip.
-By expanding to ~1/2thou over cal, springback will provide plenty of bullet grip without excess seating forces. This allows best seating depth consistency.
You can pick a bushing that leaves the ideal interference fit without further expansion action, but this leaves greater seating force variance, especially over time. So it's best to pre-expand necks before seating.
Hope this makes sense..