Enter INVASION as a password to unlock this option on the "Gallery" screen. The Enigma Machine will flash green to confirm correct code entry.
In the main screen enter "ShotGun". If you entered it correctly you will have all weapons and when you go to fight press Pause and push L1, R1, Square, and Circle you will then have all weapons.
Glitch: Moving Corpse
Go to the Nazi campsite (with the fire) on the third level in the second mission. Kill all of the Nazis on the ground. Then, go to the tent, destroy all the boxes, and get the items. Turn back around to face the way that you entered. There will be another Nazi up on a cliff to the right of where you came down. Use sniper rifle and shoot him in the head twice. Go up the hill that you used to come down. While on top of the hill, use your sniper to look at where you killed the Nazi on the cliff. You will now see his legs moving all around in the air.