I disagree with HH on his comment about pureness. You want as pure as you can get for your purpose.
Rifleman, I've been casting for bp for 42 years. 99.5% pure is plenty soft enough for any bp use. All I've used for my bp casting for the last 10 years has been stick on weights. I've cast minies for two three band Enfields, round balls for a .50 and a .54 Hawken, round balls and conicals for a number of .36 and .44 revolvers. Not to mention three 44-40's and a few other various bp cartridge loads. I'm pretty sure I know what I'm talking about.