Majority of the missions within ASSAULT GUNNERS HD EDITION unlock as part of natural progression, however a small number of missions can only be unlocked by meeting certain requirements - all of which are listed below.

Mission 21: RARE METAL JACKET: In order to unlock Mission 21: RARE METAL JACKET the previous twenty missions of ASSAULT GUNNERS HD EDITION have to be completed and then Mission 5: NO MAN’S LAND has to be replayed. To successfully unlock Mission 21: RARE METAL JACKET the Boss within Mission 5: NO MAN’S LAND has to be defeated before your remaining allied ship is destroyed.

Mission 26: PHASE VI: In order to unlock Mission 26: PHASE VI the previous mission, known as Mission 25: MISSION TO MARS, has to be completed by destroying a gate, which is highlighted with an aqua blue dot on the radar, as opposed to escaping the facility after planting the bombs.

Mission 28: GHOST PROTOCOL: In order to unlock Mission 28: GHOST PROTOCOL the main twenty missions of ASSAULT GUNNERS HD EDITION have to be completed and then Mission 15: THE GREAT ESCAPE has to be replayed. To successfully unlock Mission 28: GHOST PROTOCOL the bosses within Mission 15: THE GREAT ESCAPE have to be defeated before your allies withdraw from battle.

Mission 29: TO BE OR NOT TO BE: In order to unlock Mission 29: TO BE OR NOT TO BE the main twenty missions of ASSAULT GUNNERS HD EDITION have to be completed, then Mission 27: TOTAL RECALL has to be completed without destroying any of the terminals and then Mission 18: THE KEEP has to be replayed and completed. Doing these requirements will then unlock Mission 29: TO BE OR NOT TO BE.

Mission 30: THE CRUEL SEA: In order to unlock Mission 30: THE CRUEL SEA the main twenty missions of ASSAULT GUNNERS HD EDITION have to be completed, then Mission 29: TO BE OR NOT TO BE has to be completed without defeating the boss that appears. In order to do this simply avoid the boss until the 120 second timer has run-out; after which the eclipse will end, and you will be tasked with destroying all enemies on screen. Simply finish the mission and Mission 30: The CRUEL SEA will become unlocked.

Mission 31: THE THIN RED LINE: In order to unlock Mission 31: THE THIN RED LINE the main twenty missions of ASSAULT GUNNERS HD EDITION have to be completed, then Mission 29: TO BE OR NOT TO BE has to be completed by defeating the boss that appears within the time limit.