Sep 27th 2011, ID#4211 Unlock Special Weapons
When you complete the following tasks the corresponding weapon will become unlocked.
Unlock Infinite Rocket Launcher:
Complete the game.
Unlock Chicago Typewriter:
Complete Separate Ways mode.
Earn a five star rank for each Mercenaries mode level.
Unlock P.are.L. 412:
Complete the game under the Professional difficulty.