Go to the skirmish level selection menu. Hold [Left Ctrl] + [Right Alt] + [Page Up] for about one second. Go back to the main menu, and then go to "Arm Center." You should have every part and weapon available. This also unlocks every level and round in skirmish mode.
Big Heads
Press [Enter] during gameplay and type: give me a big head
Then press [Enter] again. To return to normal size, press [Enter] during gameplay and type: get my head back
