If it's older shot (dry to the touch) and does not have the graphite coating (greasy feel) it will work fine, probably as is without adding anything even at high velocity IF the bullets fit properly.

If the shot has a graphite coating, it will melt but is very messy to deal with and there is much residue to skim. To fully melt, the molten shot must be pressed against the wall of the pot with your ladle. I've done this and wouldn't recommend it. I'll gladly pay for better casting material than go through the process again.