Actually, you need to wait until the sprue has solidified. That indicates the rest of the bullet is also (probably) solid.
But, go ahead and experiment. Drop a few out as soon as you can get the lead into the cavity. It won't take many for you to get an idea of what the requirements are...
Dropping bullets into water might be seen as a 'cushion' that prevents damage, but 'water dropping' is normally used to harden bullets.SECOND QUESTION: Would it help my bullets if I immediately dropped them in a pan of water, like this gent suggested?
He also said this would make the bullets "harder", and I don't really want to do that. I'm using 20:1, and I would not want the bullets any harder than that.
However, it only works on alloys that contain antimony...which your 20:1 doesn't have.
You actually don't want bullets any harder than what you get from 20:1...and some paper patchers even use pure lead.
CM