Per the last post, your consistency of hold (grip strength on the gun, how high you hold the grip, etc.) is CRITICAL. I also found in the .44 what seems to be true that some powders, such as Blue Dot in a gun I sold, would cause stringing...
Incidentally, I rest the bottom of the grip frame on the sandbag/rest, not the trigger guard or barrel. The shooting of a long barrel off a bench, even with sandbags, can be punishing. I do much better off a bench with the shorter barrel'd gun I now have. Is this recoil/punishment possibly a factor for you? Are you being honest about it?
Best Regards.