I was thinking that too, most bb grenades are hard and made outa this sorta hard plastic looking outer shell. Getting hit by that on the head or neck could cause a serious injury. Nothing hard could get shot outa that thing.
As for the nerf with cap, would be a good idea... I wish we knew how to make that ninja puff smoke stuff, then it would make a bang when it would impact, not hurt anyone and be sufficient for everyone to know whos dead and whos not.