Pure lead is almost mandatory for black powder casting/shooting. The reason is; black powder produces much less pressure when it fires, the bullet/ball must be soft so it can deform,(the proper term is obturate), to fill the bore. Obturation produces accuracy. The soft lead also will deform or mushroom when it hits game.
As for the diameter that the mold casts, that would have to be determined by the actual diameter of the bore. IOW, a .490 might be good for a true .510 bore with a .010 patch, or a tighter patch ball combination can be achieved with a .495 ball.
Casting any projectile using pure lead is a PITA. The lead and mold must be at least 850 degrees, 900 would be better! Pure lead does not flow well at all. Fill-out of balls is easier, but conicals can give even an experienced caster fits!
My best success with pure lead is when I use a bottom pour ladle with the spout held tight against the sprue plate. Again, the lead has to be hot as h**l, then work quickly to get the lead into the mold without it having time to cool.
The Lyman cast bullet manual is the best one on the market for cast lead bullets. Considered by most as "the bible of bullet casting".
Hope this helps, ask away if I didn't answer your questions.