I shoot BH209 in both my Endeavor and Omega and it shoots great with both of them. I use a Harvester Short Black in both guns and it works with just about what ever .452 bullet i choose to use with good accuracy.. It's not extremely tight like you're describing just snug all the way down. Maybe give the Harvester sabots a try like the Short Black and the Crushed Rib. I've see some of your other groups before so I know its not the shooter. Good luck.