If shot time is your main goal, you could just expand the chamber with some PVC pipe. Cut off the end, shove some pipe at it with some glue, attach an endcap, and done. You will have to open it and get the right size of pipe as you see fit. Increasing the size of the chamber will increase the shot time and gives you more air pressure volume as well.
If you are going for balloons, you are going to need a lot. Somewhere between 20-40, you may have to experiment a bit.
*Edit* I was definitely thinking of a K-mod here. A pure balloon PC would need somewhere between 20 and possibly over 100 balloons. 20 balloons would probably not provide enough oomph compared to other CPS, but I think it might be enough to power small nozzles.
Last edited by soakinader
on Sun Jul 07, 2013 10:31 pm, edited 1 time in total.