I agree with small patch. Take a look at the patches and see if they tell you anything. Ball, patch, and lube combo can sometimes be an issue. I had one rifle that I thought was cutting the patches when I loaded it. However, I finally saw that my short starter had a sharp edge around the OD and was cutting the patch material when I initially started the ball and patch. In this instance, the starter was very close to the bore diameter. I made a smaller dia starter that I use to load the rifles since that instance. I sight in each rifle I make before the owner picks it up. I have picked up an issue now that I am older. There is not much variation in the vertical spread of the shots, but I now have issues with the horizontal spread of the shots sighting the rifles in off a rest. I have to mentally think to either shoot with both eyes open every shot or have my left eye always closed. If I mix the procedure I seem to have the horizontal spread on the shots. My eyes have changed a bunch over the past two years, and I never really thought it made much of a difference until I started trying to figure our what was going on. Getting old is a bummer for me, but better than some other options. Having the correct patch ball combo and good control of procedures during loading and shooting are key, especially shooting a flint. Good luck, and enjoy shooting.
Roger Sells