Best casting for me is a good hot day; I do it outdoors (keeps the melt at temp better), and I use mostly WWTs with a bit of tin added for castability. It all helps the mould to fill out and cuts the rejects. Remember, any more than 5% tin is a waste of tin andd it doesnt make a bullet harder, its the antimony that does that. I top up a melt when the pot gets about 1/3 down.
For really hard drop the bullets hot out of the mould onto a sponge floating on a bucket of water next to you, BUT dont get any water splash anywhere near the molten lead!!!(steam explosion).