You could be onto something when you mentioned too much crimp. I started shooting mine with no crimp just to see what they would do. Not consistent. I have gone to a medium crimp and the groups got smaller. I have never seen a straight wall separate except some old 45-70 brass that had a indention in the brass from the factory about 1" up from the rim. It was old, hard brass.
All of the brass I have is new Hornady.
Good luck trying the Starline brass. I might have to get some of it if you think it is good brass for the 460.
Mike

You don't quit playing because you get old, You get old because you quit playing.