You might try weighing your conicals sometime air voids occur when casting makin some bullets lighter causing flyers.. This has happened to me with pistol and rifle cast bullets. With a large conical I can see this could easily happen. This is just another varaible.

Or up your charge more. When i first started using triple 7 in my winchester, my first outing i got 2 1/2" groups. The next time i bought more bullets i upped the charge to 80 grains triple 7 (92 grains) i was back to shooting 5/8" groups @ 100 yards. Just have to fiddle around to find the right mix.