Here's what I've done to answer for a respectable mold;

Get a lump of clay...or any type of soil available that sticks together. It doesn't have to be big at all, and the one I use is about the size of a brick.

Separate it into two equal halves.

Get a walnut , or anything semi-spherical and place it on one half of the clay. Take greased aluminum foil and place it over the walnut greasy side down. Mash the other half of clay onto the works and separate. Remove the walnut.

Let both halves dry hard.

Smear the inside of the cavities with Crisco.

Make a small ball of clay just a bit larger than the cavities and place it on half of the mold.

Press the other half on top of the works.

Open the halves and remove the "bullet". Roll it around a few times to work down any irregularities.

Let it dry, and if you'd like, bake it next to a fire or in your oven.

While not authentic, before long you'll have a bushel of bullets that are in all respects spherical, and uniform in both shape and weight.