I have a Ruger MKII 30-06 that WILL NOT group worth a damn using R-P cases; switch same load but Win cases: sweet! From 2.5"-3.0" to .75"-1.25" Go figure.But there it is.
I suspect that how accurate a particular brand of brass is in a given rilfe has a lot to do with how the rifle's chamber and the neck thickness of the brass matches the die you're using. I have a .308 that dotes on WW brass sized in a garden-variety RCBS FL die. Just so happens that I get the perfect amount of shoulder bump (zero) with the die just touching the shellholder, and the neck thickness is such that the expander ball just barely kisses the inside of the neck on the way up.
This gives me very straight cases and close to zero runout on loaded rounds, with no shoulder bump.
Lapua brass is another story with that combination. The expander ball is a much harder pull and runout is not nearly as good as with the WW brass. And it's not as accurate as WW. No way WW brass is "better" though.
Of course, it could be the difference in case volume too.
