I had a similar experience with my Euroarms 1858 2-band Enfield.
1. I found that the only way to make it accurate was to use the thinnest walled hollow based Minie bullet you can cast without rupturing the walls. (see item 3.) Always size the bullet for consistency.
2. I tried to duplicate the original hollow base wooden plug ( the theory being that the plug was forced into the hollow base to spread the walls into the rifling) but with minimal success. Some people try a blob of grease to hydraulically do the same thing.
3. I altered a mold by making it deeper and thinner (larger base plug for the mold) to the extent that the charge separated the skirt from the bullet, so you can go too far in the search for the perfect bullet.
Being an inveterate tinkerer, I consider all this to be an essential part of shooting these antique style arms , so as the Chief said to Josey Wales, " endeavor to persevere".
Angel Eyes