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. Note: This also unlocks every level and round in skirmish mode.
Big heads
Press [Enter] during game play, type give me a big head, then press [Enter] again. To return to normal size, press [Enter] during game play, type get my head back, then press [Enter] again.