In the level "Numbers," after interrogating Clarke, several enemy soldiers will appear at the end of the long hallway. If you can shoot all six Nova 6 canisters (two in the back, four on the sides), you can retrieve a cassette tape in the office behind where you started. Note: It is recommended that you toss a flashbang to blind the enemies so you have less trouble taking out the canisters. Once you pick up the tape, continue with the level until the part where Weaver and Clarke both jump off the roof onto a mattress. After they jump down, Clarke will ask Weaver to help him move a fridge out of the way to his first weapon stash. Once they do, go inside and look to the left. There should be a place to put the cassette.

Note: When picking up and placing the cassette tape, the game will not prompt you -- you must have seen it being moved yourself. After that, the screen will shake and the Thunder Gun will appear out of the wall. It has twelve bullets, and it is reloaded at checkpoints.