This game is just full of secrets it seems. The powerful Thunder Gun is available in the mystery box in the game’s zombies mode but little did we know that it’s also available in the game’s campaign. Here’s how to get it.
*Slight gameplay spoilers ahead. Read on at your own risk*
First, load up the mission called “Numbers”. As soon as you gain control of your player at the beginning of the mission, there will be a bunch of enemies and six canisters filled with Nova-6 gas in the hall in front of you. Shoot or throw grenades at the canisters to unleash the gas. When all six are destroyed, look around at the very start of the level for a desk with an ancient computer (hey that was the height of computer technology back in the day) and a cassette player.
Now it took me a couple of tries so there might be a time limit or something or else the cassette player won’t open. The screen will shake and the cassette player will pop open if you did it right. Now grab the cassette and move up onto the roof. After jumping across the roofs, your squadmates will move a fridge for you. To your left will be another cassette player. Insert the cassette you grabbed earlier and voila! Thunder gun!