Thunder Gun help?

Select the mission 'Numbers.' At the very beginning when the room is stormed and Nova Gas is thrown in, kill all enemies that enter the room, quickly. Then run behind you and around the counter (next to where the intel is) and look at the cassette player. Hold 'X' while looking at it closely until the screen shakes. After you finish that, proceed on further into the mission. At the very first room full of weapons, which is after you jump off the roof and land on the mattresses, you will find another cassette player. Hold 'X' on this one until the screen shakes again. Look up and on the wall and you will find the secret gun, the Thundergun. It's identical to the one found in the zombies gamemode. From there on, every safe room with weapons will contain more Thundergun ammo.

Doesn't work, at least not with these directions. Did it 118 times, no shaking, no cassette.
If there is a way, there is another trick that That1Person has left out, or didn't know he did.

Dont shoot the gas tanks. At the start of the mission, throw at least two gernades. Then if the enemies die, you should have exploded the tanks. If they didnt die, kill them and check for the cassete anyway. It should be there, I did it on my first try.

Go to the mission then throw all your grenades at the tanks then go to the back room where the casette tape is and then hold x or whatever and then head up the ladder and go to the first armory and shove the casette in and it will go down and the screen will shake and the thundergun will appear.

Select "numbers" After the torture sequence with the doctor dude, immediately start spamming all ur grenades down the hallway. NOTE: you must blow ALL the tanks of nova six up with the grenades (fact: doing this can get u frag-master as well, the grenade(s) are what cause the nova six to leak out killing everyone, so technically the grenade kills the 5 people in the hallway) The casseste (cassesstte?) whatever thing will be open its easy to tell if it is. Hold X by it for at least 3 seconds for there is no notification that u picked up the tape. Continue on through the mission until you get to the part where theres is a weapons stash behind the fridge. immediately to the left in the armory is a table with another cassesstte player on it. Walk up to it and press X again. Then hold X until the the cassesste tape holder is completely close. You'll hear some beeping and then the screen will start to shake. The thunder gun should come out of the wall right above the table

do what That1Person says but in the beginning shoot the people and all the gas canisters.