That is still great shooting and excellent velocities. Notice with the BlackHorn 209 how the velocity increases at the fouling gets more and more. I think that is why when I was shooting BlackHorn 209 the first three shots were not as tight as say the last three shots of the day.
I think that MK85 and that conical would put a world of hurt on anything you want to shoot.
If you're looking for a .503 bore... do you have a fast twist GMB?