Explosive rounds ruin perks such as outlaw and others that only proc on precision hits. Basically, explosive rounds are better for mobs and people who are unable to consistently land headshots. If your enemy is Vex or Cabal, they will provide a damage increase. That being said, here is an updated Reddit post. The old version was apparently incorrect: http://www.reddit.com/r/DestinyTheGame/comments/2sadia/sga_explosive_rounds_and_crit_multipliers_with/ So hopefully that clears stuff up for everyone