Standard paintballs are .68 caliber. Shot gun slugs are .72 caliber, so very similar in size. And with the fins molded into it, it looks and acts similar to a shotgun slug as it spins and stabilizes.
I know the person who developed the paintball side of the FN 303, both the internal regulator etc, and the specialized round. It is probably the only reason I know about this obscure thing.
See above.
