For what it's worth, I had an original Lancaster oval bore double rifle in .50. It liked the REAL bullet by Lee and 75 grains of 2F. At 75 yards, three shots from each barrel would shoot a single 2 inch group. Perfectly good for deer. For those of you not familiar with the oval bore, it wasn't rifled conventionally. The bore was .015 oval shaped (.500 one way, .515 perpendicular to that measurement). The oval shape twisted, just like rifling. Not sure how it was cut. Lancaster had a patent on it and offered it in different calibers.
Anyway, as another poster said: too much powder, or to light of a ball/bullet the impact starts to go wide. The recoil isn't centered on the stock line. The left barrel swings the gun a bit to the left, the right barrel swings the gun a bit to the right. Find a good powder charge/ball or bullet combo, and it should print nice groups at 75-100 yards. Very nice gun. Have fun, and show us your groups when you get it figured out.