Here is an idea I came up with, basically you have a nerf type rubber/foam material neon in color so you can find it easily or the player that is out can be nice enough to pick it up for you.
It works on a simple principle, there is a compression cap in which a Co2 cartrige sits in the front, as soon as it hits on the ground the cartrige moves up in to the air seal and chamber, the main chamber punctures a hole in the cartrige and the cap mechanism locks in place thus all the air in the cartrige has only one place to go, up the chamber and through to the sub chambers.
The sub chambers would be constructed of a really heavy duty nylon tube so the BB's sit in there snug, the massive amount of pressure will force the BB's out as soon as enough pressure is built up inside the main chamber, the only problem I can see with this is if the BB's are too snug on some of them and others are not they might not release so you would only get a few coming out but I dont see this as a problem if they are not too snug.
Here is a simple design done on ms paint
The design is a simple cross section, in reality the chambers would be spread out in a spiral all around the mortar round. the Bb's would fly out the sides and hit whatever is near by, the range should be about 15 feet with .12 gram BB's.
The front would be the heaviest part so in flight the round would want to land front first, the thin hollow aluminum tube can be filled with baby powder or something else so it would have a cool launching effect.
Let me know what you guys think, I am too lazy and busy right now to build such contraption but if someone else wants to do it then go ahead
Although this round looks heavy I would estimate it shouldnt weight more than a battery pack, and the soft rubber would provide a cusion if it was to ever hit a person, the front rubber cap would be a dense but soft rubber of some sort.