I'd get a big target with a small aiming point and see if it's grouping or not. Sometimes the sights can be way off!
Most pistols are meant for PRB loads. You might try adifferent patch thickness and see if changes groups, too.
My Lyman Plains Pistol is accurate with both PRB and short conicals (Lee REAL), but I'm not certain of the rate of twist. I did switch from the factory sights to f/o sights and groups improved greatly.