Styrofoam cups and BB's.
End of discussion.
You're not going to come up with a disposable "revolutionary mortar round" that will be cost effective... and no one wants to go retrieve mortar rounds.
Also, for anyone that's interested I'm able to build these things in a wide assortment of configurations... I'm currently working on one that uses the AI propane adapter and pressurized propane as a propellant
Just need to sell the old version first to make enough funding for the new one.