Instructions
Open the "config.cfg" file in the "Players" directory of the game folder using a text editing program such as Notepad.
Add the following line to the end of this file: seta thereisacow "1337" (keep the quotation marks around the number 1337).
Add the following line to the end of the file after "1337": bind (key) "give ammo" (keep the quotation marks around "give ammo." In place of (key), use a key on your keyboard such as the letter "W" to create "bind W "give ammo." This will bind the cheat for extra ammunition to the "W" key for use in the game.
Save this file. Right-click on your mouse, choose "Properties" and set the file to "Read Only."
Start the game and press the key to which you bound the cheat to activate it while playing a mission. Anytime you run low on ammo, activate the cheat again to get more bullets.