I'm not sure what I cast mine out of, it was scrap from a scrap drum scale they trashed at work. I don't see that it needs to be pure lead, though, and I don't think what I used was pure because it didn't work with my 360 grain Minie, tried it. Those Minies work great with pure lead. I do need to get a hardness tester.
But, as there's no skirt to expand, I don't see that it has to be quite so soft as a Minie does. These things were tack drivers on my CVA Wolf (1:28 twist). I'm quite happy with that as when I bought the mold, I tried 'em in my 1:24 Hawken Hunter Carbine and they didn't work for crap. I haven't tried a filler over it, never thought to, but in that rifle, I don't think I need it.
That Hawken Hunter Carbine is real picky. It won't shoot 777 or especially APP worth a toot, but works great with Pyrodex. Go figure. It likes the sabots and the Hornady Great Plains Minies and those Lee 360 grain Minies if I can find pure lead for 'em. The CVA Wolf seems to be much more amiable to various loads and bullets. It likes the 777 load, 80 grains. I really like 777 because it has no sulfur. I understand it's still corrosive, but it's easy to clean up compared to pyrodex. I can't easily get true BP anywhere within 200 miles I've looked. I could order it, but I will stick with the subs. I really like the 777 and I see very little difference in Pyrodex and BP as far as how they perform and smell and everything else. I don't understand the hatred of Pyrodex by the purists. Give me a gun loaded with Pyrodex and one loaded with BP and I couldn't tell the difference. The ONE thing BP does best is it's easier to ignite. If I could get it, i'd probably prefer to shoot it in my Plainsman with the number 11 percussion caps. I get a hang fire now and then with 777. If I keep the nipple clean, it works well, though.
From what I can see, every gun is an entity unto itself. It's hard to predict what will work in one gun or the other, just have to shoot it and see. I can make rough predictions like 1:66 won't shoot a conical or 1:28 won't shoot a patched ball, but beyond that all bets are off.