Other models of gas guns and helpful information

My first ideas on how the model should work generally are correct, however, some verification or additional ideas are always helpful. That's why I've looked into other models. Some I've looked at are detailed in the links and descriptions below.

Spud gun models

CALM - A simple isothermal model that is also open source.
GGDT..... The Source Code - This code represents GGDT as of Jan. 2004, and if you can follow my code you should be able to follow most of D_Hall's.
Apocalypse by Ragnarok - Not publicly released. Takes a 1D approach to the problem like many of the models listed here. Uses the method of characteristics. For more, ask Ragnarok.

Books (and book length reports)

Theory of High Speed Guns by A.E. Seigel - Describes an interesting method of characteristics based approach to solving high speed gun problems of any variety. Widely cited. I intend to print a copy for myself.
Interior Ballistics of Guns, edited by H. Krier and M. Summerfield - A more modern (1979) take on interior ballistics problems. Contains a chapter by A. E. Seigel that seems to be a short updated version of Theory of High Speed Guns.
Nonsteady duct flow by George Rudinger - This is a long out of print book that describes the method of characteristics applied specifically to problems of 1D duct flow. Consequently it is very applicable to gas guns (indeed, interior ballistics is one of the subjects mentioned in the introduction). This book is cited by several of the papers listed below. If you intend to use the MoC to write a simulation, this would be a good book to have. This book can be found used for about $20.
Nonsteady, One-Dimensional, Internal, Compressible Flows: Theory and Applications by John A. C. Kentfield - Another excellent book that is no longer in print. Makes heavy use of the method of characteristics, but finite-difference methods are mentioned as well. Covers the entire range of applications of unsteady 1D compressible flows.

Other simulations and theory (mainly for light gas guns)

The Light-Gas-Gun Model Launcher - Goes fairly in depth into the theory behind light gas guns. Uses the method of characteristics. Written by NASA.
Theory of transonic gas guns - Another interesting report by the Australians. This one deals primarily with analytical (i.e. plug the numbers in and get a solution) methods.
QUICKGUN: An algorithm for estimating the performance of two-stage light gas guns - This paper describes a simulation of a light gas gun and includes some equations. Uses the method of characteristics.
A Numerical Simulation of Gas Gun Performance - I haven't examined this one too closely but it looks interesting.
New higher-order Godunov code for modelling performance of two-stage light gas guns - This is likely the most advanced simulation of all here. This one does not use the method of characteristics, rather, it uses Godunov's scheme, which is entirely different.
Internal Ballistics of a Spring-Air Pellet Gun - This model takes an energy based approach to the simulation.

Requisite knowledge

A simple model can be written with knowledge of basic programming, numerical methods for ODEs, thermodynamics, and control volume analysis. However, these models are sorely inadequate for transonic velocities.

I can not offer much more than I have to those wishing to write their own models of transonic gas guns. My suggestion would be to be knowledgeable about all things mentioned for the simple model as well as gas dynamics, and in particular numerical methods for gas dynamics like the method of characteristics and/or some finite difference/volume schemes. This however is graduate level engineering or applied mathematics and is NOT something most can handle. If you are willing however and have access to a good library or much money to spend on books, it is possible.