If you want to cast and roll your own, why don't you just cast and load slugs. It's a better projectile in every way. Round projectiles "knuckleball". That's why pellets are more accurate than bb's out of the same smooth bore barrels. You'd be giving up accuracy for a slight increase in weight. A slug is lighter than a ball but the sharp edges make "better" wounds. A slug also penetrates farther.
When smoothbore muskets were used for warfare, you were pretty safe from someone that was aiming at you at +50 yards. That's why they used massed, volley fire.
I don't see any benefit. If you are going to buy a mold anyway, why not get one that'll do slugs?