I have shot a considerable number of round ball bore rifles over the years and I have always found that a melt between 1:20 and 1:10 with a groove diameter ball gives the best accuracy. Softer melts like 1:30 are great for hunting but a little bit more accuracy can usually be achieved with a little higher tin content. All this being said I have also found that all of these rifles are essentially unique so I would try several different melts and let the rifle tell me what it likes!