Its going to take time for you to stop worrying about that flash in the pan going off so near your face. It looks like you are "pushing " the trigger a bit. The 3 shots at 25 yards should be closer. Increase the powder to 75 grains, and try it again. Then go on up to 80 grains. You should see some improvement in the group size with the slightly higher velocities. Also, as the barrels settles into the stock, and you become more accustomed to shooting a flintlock, I expect your groups will shrink.
Do measure that bore, to determine its exact size. Also, then measure your patching material. Because this is a smoothbore, you won't be able to use as thick a patching as you might if this barrel were rifled. Compression of the patching becomes a factor in determining group sizes with these guns. If the balls are pure lead, they will obturate in the barrel to seal the patching between the ball and the bore, compressing the patching further.
Also, consider buying some 60 gauge OP wads, and try them. That also reduces group size radically in these smoothbores. I believe this is because the OP wad delays the movement of the ball out the barrel for a few milliseconds, allowing chamber pressure and temperature to rise. This allows the powder granules to burn more completely, and for the velocity from shot to shot to be more uniform with the same powder charge.
I want my gun to be shooting balls into one ragged hole at 25 yards off a rest, and into a 2-3 inch group MAX. at 50 yards. I believe that these modern steel barrels are capable of that level of accuracy, if you develope the right load.