You might need to investigate if you think the sights have been modified over the years. At first blush it would say you front sight is to low, however that does not rule out load like bullet weight speed, etc. I personally would wait until you could try the rifle at 100 yards. I say this because the math to correct is easier if done on 100 yard results. (Just my way of mentally calculating solutions. smile ) Generally thinking load and speed are not the major contributor to the results. Physical sight heights being that major contributor.
Shoot the rifle a few more time as it is and get a better understanding of what is really happening here and then make corrections as needed.
As you may see I also am working with one of these great 2 barrel rifles. So much fun. Can't hardly wait till winter breaks and I and my good friend can shoot mine. Maybe if it shoots really well he will take it on his Elephant hunt he is going on this spring. LOL smile just for you Rick.