Marrakai,
about half the doubles that I have owned over the years shot high such as yours. This indicates that the projectile is in the barrel to long. A little higher velocity, sometimes a change in powders usually made a difference. Graham Wright covers this in his excellent book/s.
The last 2 guns that had this problem were a Westley Richards .450 no 2 that belonged to a friend and a P. Webley & sons .500 underlever hammer gun that I sold recently.

Both guns responded to an increase in the loads. (being careful to watch for pressure, of course.)

cheers