To unlock the dual chain gun, first select "Pickups" at the bottom menu. Scroll through items, then select the locked item, and enter "MEGASHOT" as a password.

ACHIEVEMENT - HOW TO UNLOCK
Entirety (30 points) - You must play through all levels.
Hard to Complete (220 points) - You must complete all levels on the Hard difficulty setting.
Immortal (100 points) - You must play through all levels without dying in any level.
Cosmic Combat (10 points) - You must play through the entire Cosmic Combat level.
Deep Sea Danger (10 points) - You must play through the entire Deep Sea Danger level.
Emperor's Tomb (10 points) - You must play through the entire Emperor's Tomb level.
Graveyard Gunfight (10 points) - You must play through the entire Graveyard Gunfight level.
Insect Invasion (10 points) - You must play through the entire Insect Invasion level.
Robot Rebellion (10 points) - You must play through the entire Robot Rebellion level.
Third Dimension (10 points) - You must play through any level in 3D.
Single Shot (150 points) - You must play through any level using only the single shot weapon.
Staying Alive (80 points) - You must play through and complete any level without dying.
100 Ant Kill (50 points) - You must kill 100 ants in one play through of Insect Invasion.
Reactor Destruction (50 points) - You must destroy all the ship reactors in one play through of Cosmic Combat.
Underwater Mine Expert (50 points) - You must destroy all the mines in one play through of Deep Sea Danger.
Bottle Shoot (50 points) - You must shoot all bottles in Graveyard Gunfight.
Mine Expert (50 points) - You must destroy all mines in one play through of Robot Rebellion.
Hotshot (25 points) - You must get a hit streak of 20 shots in any level.
Big Score (25 points) - Earn a score of at least 75,000 points in any one level.
Stolen Idol (50 points) - You must steal the Idol from the Emperor's Tomb.