I recently came across this buckshot mold I made almost thirty years ago. If I recall correctly the plan was to use the buckshot for shooting geese, but I never got around to actually use any of them.
As you can see the principle is quite simple, it's based pretty much on the system used in plastic injection molds, with a central runner and a small inlet into each cavity. I have only tried the mold with a Lee bottom-pour, but it would probably work with a ladle as well.
The size of the pellets is approx .25" - not quite #3buck but larger than #4buck. 16 of these (one full cast) makes for an even ounce. The idea was to have 4 layers of 5 pellets each for a 1-1/8 load with a felt wad & buffer.
Separating the pellets from the runner is quite easy. When cast in lino a slight squeeze will snap the inlet from to the pellet. Cast in pure lead it takes a little more effort, but not much.
The mold was made on a Bridgeport mill, but if anyone should be interested in making his own mold, a drill press with a table with X & Y movement should do fine. Use a two flute ball nose mill for drilling the cavities, and a dremel tool or similar to grind the inlet. Make the inlet larger toward the runner to insure the sprue will snap flush with the pellet.