Mr. Hamlyn:
If I may be so bold, I would suggest that before you start your journey into Black powder Cartridge, that have a look at several of the books written by Paul Mathews. They are a wealth of good information, by someone who has actually figured these tips out the hard way. He addresses a lot of issues like alloy, lubrication, bullet molds, shooting, and loading practices.
I used to have several of his books, and in a moment of poor judgement, sold them on the Exchange forum. I now have a rule that I will never sell reference material again, as you never know when old interests will flare up again LOL.
If memory serves, (it has been some time) I believe that Mr. Mathews found that softer than normal alloys worked best for him. He started with a 1 in 30 ( 1 part tin to 30 parts lead) mix, and slowly increased, and after much experimentation, ended up working a lot with a 1 in 20 mix.
I suggest you google him to find out the titles, and refer to Amazon.ca as I believe they carry those books.
Regards and all the best
R