Actors are frames, because the frames get animated. So, link the weapon's mesh to a box, and name the box "frame gun". Don't collapse to editable mesh or apply a material, just leave the box as a primative

For example, here's the Rocket Launcher third person:

frame gun
  __unnamed (the geometry)
  #ground point
  #left hand cyborg
  #left hand marine
  #primary ejection
  #primary trigger
  frame body
   frame push_bar
  frame foregrip
  frame tubes