I think a vent at the rear would be needed for 2 reasons

Firstly to prevent recoil. Due to all those physicy things like equal and opposite reactions and conservation of momentum you need the gasses to be expelled from the rear of else the whole launcher would be pushed backwards. Secondly, you need a vent to stop pressure build up.. This is why rocket launchers and recoiless guns etc are much lighter than the equivlent conventional weapons, as they are basically (comparatively) thin metat tubes whereas a normal gun has to contain the pressure of charge used to propell the shell.