This is the easiest way to do it:
1.Download the fragmotion trial.
2.Take any weapon that is in fpsc and import it into fragmotion.
4.Export from fragmotion into a .ms3d file.
3.Open the weapon in milkshape. Line your weapon up with it, then animate your gun. (Make sure to delete the old gun)
4.Save as .ms3d.
5.Open your animated gun in fragmotion.
6.Check to make sure the verticies aren't messed up. If they are, assign the messed up ones to the right bone.
7.Export as a .x file.
8.Use the gunspec of your template weapon for your gun.
That's it, I won't be around for a week (unless I can reach a computer), so I won't be able to help if there's problems until next Wednesday. If you leave a question, I will answer once I get back.