There seems to be a difference between ball-bearing and BB cannons. Apart from the size, what else?
Ball bearings have a lot more kinetic energy when fired, which causes more damage on impact. Ball bearing guns also use a simpler gun design because the depend on the “flow” of gas to fire the ball bearing. BBs don’t have as much power, which makes them safer for spectators. BB cannons also use the “pressure” of gas to fire the BB.