Here is a ammo mold I made from a socket for slingshot ammo it is deadly on small game with the 1/4 socket you could make some good buck shot for shot guns and with some research shot gun slug also.
It might be interesting to note that shot originally made by dropping molten lead from a tall structure ,called a shot tower, into a vat of water . Lead will naturally form almost perfectly round in the fall and partially harden before hitting the water. I have a bag of lead BBs made in the turn of the century using this process. Making and Using a mold out of plaster of paris is not all that hard, and might give you better performance in energy accuracy and material spent.
Smaller Shot 6-9 is now made by the bliemiester method. (Molten drop lead into cooling bath and incline. ) the most accurate shot at longer range is then chilled in a cold bath, which causes the lead balls to harden, and deform less under stress. Larger shot is made with lead bars fed into a crimper and pinched off. The dust on your fingers when handling shot is added graphite to prevent clumping.
Great idea! Are they hard to get out of the mold after pouring, and do you smoke the socket on the inside before casting? I could see a row of these mounted on a board, and pouring 5 - 10 at a time for a gang mold effect. I see a lot of the smaller sockets at garage sales and in pawn shops for not a lot. My favorite flip ammo is .36 lead, but I only have a single cavity mold at present, so I would probably like these in a 5/16 socket.