Here is the full list of all 19 Railgunners achievements.
Beat the Training Grounds stage
Beat the Pirate's Bay stage
Beat the Tropical isles stage
Beat the Oil Route stage
Beat the Dragon Peaks stage
Beat the Dark Skies stage
Beat the Storm's Eye stage
Beat the Autumn Rivers stage
Beat the Castor Lake stage
Beat the Volcanic Caves stage
Beat the Sleepless Desert stage
Beat the Cornucopian Fields stage
Beat the Mother Forest stage stage
eat the Canyon maze stage
Beat the Pinnacle City stage
Beat the Ground Zeroes stage
Equip your first weapon
Equip your first mod