Based on my own observations, I think loaded round straightness is a big factor with the 30's. When I first put my 30BR together, It didn't shoot quite as good as some of the others hanging around the range. I was using a popular brand seater that is common in Benchrest.
Jackie: I may have posted this before, but the neck in an unmodified Wilson seater for the 30BR will measure between .335-.338. Since most chamber necks are .330 and most guys are running loaded rounds in the .328 area, it's not hard to see that the necks are not well supported during seating. When the neck tension goes up, things can get pretty hinky in runout-land. Jamming hard covers up many of these woes, but that doesn't mean there not still there. I know what we do to correct this and you've posted your fix.
Something to look at....... -Al