re: BLO

A spray lacquer works,  You'll obviously have to mask off the non-wood areas.  A gloss wiping poly might be easier.  It is thinner than lacquer so is less likely to show brush marks.  It just takes more coats to build up.  After putting on several layers of finish, you can use a polishing compound or sanding wax (like Acks) to polish out any finishing defects.  I do this with turned items with both lacquer and poly.    

But personally, a gun stock pen should have a more satin finish as a real gun stock would. My two cents.