In shadow result with IBL lightning. Looks pretty dull, perhaps in life this will look as it is, but the game we would like to focus on weapon details even in shadow.

As an idea to create a special light source that will affect only specular and  use it in the shade.

diffuse_light = light_accum.diffuse * light_diffuse_factor; //diffuse influence factor
specular_light = light_accum.specular * light_specularfactor; //specular influence factor

 

It’s simple to create and easy to use.