This sounds like a very interesting problem. What you can do is model a body at the location where you will have an initial condition of high pressure in the barrel of the gun. Then, within the flow simulation software, you can take that body out of the computational domain through component control. (Right click on the tree, input data, component control.)
You'll still be able to see these bodies and select the bodies, it's just that the bodies are no longer part of the solid computational domain. When you make you're selection for the initial condition of pressure in the barel, you should still be able to select that body.
All the best,
Nick LuysterOnline SolidWorks Simulation Training
Local initial conditions sounds like the way to go but seems like overkill. Why not just use a transient analysis to create a flow pulse with a volume flow rate?
Time steps are going to have to be really small for this to work well.