Original design, which was a golfball, was posted in GrabCad by "Bass"https://grabcad.com/library/i-made-a-golf-ball (Recieved Permission to Post on site)
Suggest printing with TPE in order to make them as soft as possible. Also best to work the pellets with your fingers for a little bit in order to break up the infill.
They do pack quite a punch! Goggles recommended! I only allow my son to shoot them at me, otherwise they are used to hit targets outside.