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