If you are low level and not seeing it drop, try leveling up, and keep playing through, I am level 60, and since about 45 I think it was, I have been getting those rounds like crazy. fair warning by the way, you can only shoot like 3 shots from an assault rifle with those, even with fictionless materials. Alternatively if you own Pinnacle Station(Downloaded Content) you can spend 220,000 credits for random item shipments after fully clearing the mission from your appartment on Intai'sei. Every purchase has a chance of producing High Explosive Rounds X.