Getting the Rail Gun early.
When you enter the sewer and have the cutscene with Echo, when the shooting starts, equip either the Nano GL or NICW and run towards the tunnel straight in front of you where the 'beam' comes from. Just before you reach the opening start firing grenades. Do NOT use the timed grenades (Nano GL). If you time it just right, The enemy will have dropped a Rail Gun and an HMG.
I found this one by accident during a playthrough.
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