In Mission 7: Numbers, after interrogating Clarke, several enemy
soldiers will appear at the end of the long hallway. If you 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
can shoot the canisters more easily. Once you pick up the tape, continue
progressing through the mission until the part where Weaver and Clarke
both jump off the roof and 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 to do anything
-- you must use this information to know when to do these things. Once
you insert the cassette tape, the screen will shake, and the Thunder Gun
will appear out of the wall. It has twelve bullets, and it is reloaded
at checkpoints.