Woodsman you have come to a sort of fork in the road! Do you want to darken the finish that is already on the stock or do you want to darken the wood? To darken the finish that is already on the wood the application of a coloured Lacquer or Varnish will work though to be honest not a thing I would do because it always looks as if it is lacking something. Applying Linseed oil to wood that is already finished with an unknown would not normally darken or change the looks of the wood Linseed oil is only comfortable being applied on top its self or direct to wood, otherwise it can be prone to blistering or wrinkling if applied over other finishes.
Stripping the stocks original finish down to the wood and applying coloured wood stain and a gunstock final finish of your choice would bring with it a far finer final finish to the guns stock.
