Let the mold cool on it's own. Cooling the sprew will not allow the bullet to shrink as it cools and pull in hot metal from the top to fill the void.
To get excellent bullets, you should overfill. In other words, let a stream of metal pour over the top of the mold and back into the pot even after the cavity is full. Let it flow for about 4 seconds and then let the sprew cool on it's own.
As I said, it slows you down, but you still get more GOOD bullet every hour, because you don't have rejects.

Steve