I studied this as I was reading the Horus heresy series. Did you know that the armor the loyalists were using on Istvaan V (the huge massacre when Horus showed his colors and when Fulgrim killed Ferrus Manus) was sabotaged and weakened before the heresy even started? Horus infiltrated the the forges on Mars. The Raven Guard discovered that Horus's forces had armor that would stand up to bolter fire, while everyone who came to Istvaan wore armor that bolts shred easily. There are some armor sets that resist bolters fairly decent, but nothing but a dreadnought or Titan can stand up to sustained bolter fire.