Select the 'Pickups' option at the bottom of the screen and Scroll through the items until you reach the locked item. Then enter the case sensitive password 'MEGASHOT' (without the quotes) to unlock the dual chain gun.
Megashot - Dual Chaingun item
Select "Pickups" from the menu at the bottom of the screen. Scroll through the Pickups until you reach the Locked Item. Select it and enter MEGASHOT to unlock this gun.