Have you looked at the BPI sabot componants for shotgun slugs? I'm having a little sucess (<=2moa) with the 12ga x 50cal sabot and 350gr 50 cal Hornady. I'm using 28-29gr of IMR PB powder. gas seal. roll crimp. I'm also struggling to make a clean roll crimp. I think I'm going to start trimming the hulls so there isn't so much material to roll over.