The Soulstorm Rifle is really really weird now that have seen it in full power.
without any upgrades it does around 24 to 26 damage to any enemy type including bosses, but if you apply even a single artifact, Bosses only get damaged upto a max 9 points per hit compared to 26 before.
other enemies receive increased damage normally of course
but the weird thing is after applying Firestorm or Sniper mod which both alter the projectile of the rifle to bolt and laser the bosses also get full increased damage as the should from each projectile along with 3 Artifacts upgrades.
this means that applying Artifacts on the weapon without mods nerfs the weapon against bosses of any kind. (nano mod's reloading half health sap works on bosses wonderfully)
and there is a bug with the Marine Class in which after applying the 3rd Artifact and either Firestorm or Sniper mod along, the rate of fire gets reduced massively low.
and as stated in a post before, the Firestorm mod doesn't act like its description says.