No idea if anyone's still playing this game or reading these forums. Couldn't find a suitable answer on the official forums for the game.
What is the "secondary fire" button in the game? There doesn't seem to be anything in the keybindings that is listed as secondary fire so you can't bind it and there doesn't look to be a default bind.
When I saw the hint that says what it is, it says Q, but when I bring up the assault rifle with a grenade launcher that's supposedly fired with secondary fire, pressing Q just makes me throw my grenades instead of launching a grenade. I can verify this by just switching to molotovs.