I dont have any experience with this type of gun (if its what I think it is) but I am interested to learn.

It would help me out to know what your gun is? Is it an old black powder single shot? A Double?

The 8 inch groups at 25 / 50 /100 yards?

Are the balls little bigger than the bore? This is smooth bore? Rifled?

If this is an old gun, I wonder if it might be better to begin with black powder and try smokeless later?