Sam, this link seems to have a formula which may work. To run it I would need to know the original barrel length and velocity, the shortened length, and also the cartridge volume. If you can provide that I can try running the calculation. I have some links on case volume, but not sure if they include handgun cartridges. If they are straightwall cartridges then they should be easy to calculate in any case if you tell me the cartridge.
http://www.loadammo.com/Topics/October05.htm
Ron