A muzzleloader enthusiast will want to cast his own lead balls, just as an 18th century minuteman did. Some steel and aluminium moulds are available from manufacturers like Lee, but these are less authentic (and far more expensive) than are simple plaster moulds. Plaster being inexpensive, you can create as many moulds as you need.
Rub your model ball (if it is metal or modelling clay) with some vegetable oil or vaseline.
Cut off the steepled top of the milk carton, which should leave an empty box with no lid. If you are making a mould for a single ball, a small milk carton is sufficient. If you are making a mould for two or more balls, use a larger (half-gallon) milk carton, and cut it to about 4 inches of height. Mix Plaster of Paris (in about a 1:1 ratio), and fill the milk carton halfway.
Press the ball (or balls) halfway into the plaster, touching one side of the carton. If it is a multiple mould, allow about ¼ inches between each ball. Allow the plaster to dry, or oven dry it per the manufacturer's instructions. (See Step 9 for a more complex method.)
Melt some wax (simple candle wax will do) in a pan. Brush the melted wax lightly on the hardened plaster and ball; this will enable the two halves of the mould to separate easily.
Mix more plaster, and fill the top half of the carton, covering the ball. Allow to dry, or oven dry it per the manufacturer's instructions.
Tear away the milk carton, and before you separate the two halves of the mould, carve or draw marks that will allow you to match up the halves exactly. Separate the two halves of the mould, and remove the ball.
Match the halves and clamp the mould together. You may have to widen the hole, just slightly, in order to pour in lead.
Attach a hinge and a clamp, if you wish, to the two halves of the mould. Use small screws and drill pilot holes, so as not to crack the plaster.
Alternative method to create perfectly round balls: Place the model ball 1/4 from one edge of the carton, and proceed as above. When the halves are dry, clamp the mould together, and drill into the seam (the one nearest the cavity) with a 3/16-inch bit, until you penetrate the cavity. Widen the hole slightly, with a larger bit and near the surface, for an easier pour. You will need to cut off the excess lead (the sprue) from your finished balls.
Because the materials are inexpensive, you may wish to cast two moulds at one time. Your ball will not be perfectly spherical; it may be slightly flattened or bulging on the side of the pour, which is perfectly authentic. File off any excess (sprue) with a steel file. A good plaster mould should last for several dozen pours, before you notice that it begins to "flash," with excess lead forming along the seams of the mould. Discard the mould and make another.