Use the longest barrel you can. Potato cannon builders generally recommend that the air tanks are about 3 x the barrel volume. Matt and Vivek also made good suggestions about using a pneumatic release and parallel sprinkler valves. We have one here at home with three 1 inch valves in parallel, converted to pneumatic release.
I don't know to what extent the balloon will squeegee off any soapy water or shaving cream as the balloon proceeds down the barrel. A more waxy or greasy lube might work better. You can do some coefficient of friction testing and let us know
Occasionally you might find a surplus pneumatic dump valve for a good price. These are used to clear blockages in dust collection systems among other things. We have a couple of 1.5" (I think) aluminum body valves that were about $15. each on fleabay.
If using PVC, always use
pressure-rated pipe, not DWV (drain waste vent). Learn to make proper joints (scuff, solvent, cement) and let them cure for 24 hours. PVC gets brittle when cold, and the ultimate strength drops off as it gets hot.
http://www.madisongroup.com/failure/...eanalysis.html