I like the ideas...it's be a great defensive setup.
People are naturally going to bitch about getting wailed on by so many shots at once...but if it proves safe (i.e. point blank safe) it'll be all good.
The biggest downside I see is directing the shots upwards...it'll go right towards any gaps that exist between the bridge of the nose and goggles/glasses (asian no-bridge syndrome). If it's directed a thigh/waist level...it's perfect.
Spring power is best....gas is too temperature sensitive/tempermental...besides, you'll want to set it back up after it's blown without having to carry around a propane tank or a bunch of shells. Resetting it with a handy stick in the MSpaint diagram above can't be much easier.
Why not rig it so that the main barrel/assembly is more horizontal and the pin is pull downward off a pulley to redirect the trip wire?
Need prototype pics!