I find ACWW work fine in black powder cartridges, and that's what I mainly use. But you need to remember that they will get harder over time. After a year or so, they do not shoot as well as when they are fresh. When I was young, an old timer taught me that if you can easily scratch/dent your bullets with a fingernail, they are soft enough to work. If not they're too hard.
You can anneal wheelweight metal in an oven to soften it back to where it was when cast if you like.