Dug around a few videos of airsoft mortars, found this interesting concept, using a M203 type grenade as a means to propel the "object". Actually is quite brilliant, have a fixed firing at the bottom of the tube, make a sabot to fit both the grenade and the "bomb" into one tube, drop it in, firing pin releases the gas and POP!
http://fr.youtube.com/watch?v=WYsVtZ9ieK8
http://fr.youtube.com/watch?v=YBIZZ1r-8-g