I shoot with a mate who has that Whitworth rifle. He has had it for less than 12 months but is quite pleased with the results he is getting with it. He runs a standard .45 calibre lube groove bullet , ( similar to those in my post above ) cast from pure lead. The soft pure lead will obdurate out and fill the hexagonal bore when fired. We have dug a few of his bullets out of the mound and found that they indeed are fully hexagonal.
As for paper cartridges, not my field. I use brass, my guns are breech loading designed for metallic cartridges.
If the metal you have is indeed "printers metal", it should be
linotype. It should have a BHN of 22. Pure lead is 5. Lead wheel weights are usually 95.5% lead, .5% tin and 4% antimony with a BHN of 9. Mixes with antimony should be avoided in black-powder rifles due to the fact that they tend to "lead" the bore.
If it has a BHN of only 8 it would certainly be soft enough, I would be casting a few up to see how they go before going overboard.
We should have a couple of muzzle-loader shooters attending this week-end, I'll quiz them for you.
