Assuming that the guy pulling the trigger has previously demonstrated an ability to shoot at 200 yards, I would guess you have a stability problem with the heavier bullets. 1:8 is a fast enough twist, I would think, but we'd have to know the velocity to be sure.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.