The real problem with this idea is the projectile, but i think i have a solution.
Make a canister, shaped like a regular tank round. have four compartments behind the head running the length of the body. Each of these compartments has a spring loaded bottom that when released springs outward. each compartment also has a door over the top that is realesed at the same time as the bottom panel. A trigger is built into the nose of the round so that when it hits a wall or another solid object the catches release and whatever is in the compartments sprays all over the poor saps next to it.
NOTE: this would not be particulalry safe becasue it could seroiusly injure anyone that got hit directly by the shell.
these would prolly cost about 5-10 $ each if they were mass produced.
could bring in the age of airsoft rocket launchers....