in short:
when you finished the model in a propper way (polyreduced, textured) you have to rig the weapon on a "FIRESPOT"-called bone to do the animation (named firespot so fpsc can place muzzleflash-decals in any form), then rig it to hands if wanted.
animate it as you wish (i have loaded a stock-x-file of a fpsc-gun with fragmotion to get the right dimensions for the HUD.x).
i am learning this myself atm, so i cant give you detailed info. when you got the basics ready, most if it is a trial and error-thing.
search for blender-tutorials by gamerX on the fpsc-board, they are easy to understand and very helpfull.