Assault rifles generally have limiited amounts of tech and large proc costs, with certain exceptions (like the Serpens and Ogre). The 1.4x ironsight with the dot in the middle carries with it a boost to tech pool, raising proc rate even higher on weapons with high proc and bringing weapons with low proc rate (like the Draco) into the medium range.
The soldier can make the most of this, using a Orion/Draco/Serpens/Gunman class that obliterates anything. Because it covers every resistance, there are no circumstances where this approach would not work.
The siren receives bonuses to tech pool and elemental damage, making the Draco or Serpens an excellent choice for her as well. It should be noted, however that the Hellfire will perform better for the siren, due to her Smg boosts