The certain members of the spudding community have written computer programs pertaining to spudguns:
Accuracy of the predictions is typcialy within 5-10% of the actual value, although there has not been much testing of very long where the (unaccounted for) flow resistance due to barrel length is significant, or very high pressure launchers where the non-modeled liquification of the propellent gas(ses) may take place.
Standing for Compressed Air Launcher Model this is a Pneumatic Launcher simulator program by AKB of the SGTC. Since it is written in the Python programming language you can read the code and see how it works. It is not graphical, but produces tables of pressure, velocity, height, etc.
CALM can be downloaded at http://www.qsl.net/wb6zqz/calm.html .
EVBEC stands for Energy & Volume Based Estimator for Combustions. Written by boilingleadbath, it is based on Latke's chamber:barrel test data.