This is how I do it. Place a camera at the origin, and set it to use a 20mm lens (I think that's what I use). Set your view through this camera, and it should give a field of view very close to FPSC. Then it's just a matter of scaling your gun model until it looks right through this view.
It doesn't have to be perfect, because you can adjust it's position in FPSC in the gunspec file. But doing it this way will get you in the ballpark right off the bat.
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