Instructions
Beat the game with the difficulty set to either "Big Boss Hard" or "The Boss Extreme." During this entire play through, you must not die at all, not kill anyone aside from the bosses, not use any recovery items, trigger an alert three times or less, not use any special items and do all of that in under five and a half hours. Doing so nets you the "Fox Hound" emblem and the Thor .45-70.
Beat the game on any difficulty to unlock a password entry option, and enter a password to get the Thor .45-70. After you beat the game, go to the title screen and select "Extras." On this screen, click on the option "Passwords" and enter "tshsniammr" without the quotation marks.
Begin a new game to utilize the Thor .45-70. Regardless whether you use the password to obtain the gun or get the Fox Hound emblem, the gun will be available after you have met the Mk II, the small helper robot, toward the beginning of the game. Press the "Start" button and select the option that says "Weapons." Here you will find the Thor .45-70, which you can assign to one of the active spots in your weapon's quick select menu. When using it, you must reload after each shot.