Plastic ball ammo for paintball pistols

My son wanted some reusable ammo for target practice with a low cost paintball pistol. 

These fly very hard when fired from a paintball marking pistol.

They do not, however, fly straight for very long.  They either need dimples like a golf ball or to be better balanced.

Hollow and solid models included.  Take about 10 mins to print on a 0.2 mm layer height.