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while interrogating Clarke, soldiers break in (You're playing as Hudson in Kowloon, BTW). Right when they break in, quickly throw one or two grenades and turn to the room directly on your right (don't go up the ladder yet). On the longest desk, there's a tape player. If your grenades destroyed all the Nova 6 tanks, the tape-deck on the tape player should be opened (the intel's in the corner if you want to grab that, too). Go up to the player and hold X. It won't say you picked it up, but just hold it for like 3-5 seconds. If the deck's not opened, just restart. Play through it and, of course, rush up the ladder and out of the building so you don't die of Nova 6 exposure. After Clarke and Weaver move the refrigerator to open the entrance to Clarke's weapons cache, go in and look to the left where Clarke picks up a G11. There's a tape player at eye-level. Go up to the tape player and hold X. You'll hear some weird sounds and then the room will shake. You'll see the Thunder Gun come out of the wall. Go up and grab it by holding X. There's 12 ammo reserve plus 2 in the chamber. One shot will nail anyone within its range and way. The only reload you get is in the warehouse where you have to protect Clarke while he's unlocking a secret door. Go into the crack in the wall where there's a Grim Reaper and all those weapons (it should be the dot that tells you where to go). It'll give your Thunder Gun another 4 ammo.
@Mustard: Yes, it's in Numbers
@Alexei: Black Operations is not all covert as you believe. Covert Ops and Black Ops are 2 different things. Try playing on Veteran, it's hell on earth