Anybody who believes a rice filled ball, even at a fast speed can cause a concussion, is beyond reasonable. The physics of such a light weight that is also grossly deformable means the impact force is very low.

Cops fire 'non-lethal' bean bag rounds out of shotguns at 800 feet per second (250 meters per second or 880 k/h). They leave a minor welt on the skin but do no damage to internal organs. Yes, they sting. But, the light weight means no energy is transferred to the target.

I'm to the point that I do not even want to address such ridiculous ideas. They are just anxiety. The startle of such hits can last if the person lets the anxiety take over.