Hmmm... I bought a bag of marbles at the dollar store. There were about 50 of them between .61 and .635" sizing was not more consistant than that. Anyway I selected ne that I thought was reasonable and weighed it 82 grains. Similar in weight to a .625 stun dart. Performance through the tube was close to half the speed of a stun dart. Watching the trajectory was easy and painful. I seriously doubt a ball shape in the tube is close to as efficient as a cone that fits as well.