Here's the first idea that I thought of.
Reusable mortar round seems mechanically simple and can be made of materials that for the most part, wouldn't hurt if hit directly. Also a direct hit won't shoot BBs into your face as they come out the back.
It would have to be fired with either some serious pressure or maybe some sort of spring system. The round can be wide enough to create a seal around it if a gas launch is used.
