Infinite ammunition in campaign mode
To do this trick, your weapon must be reloadable and have at least one clip in it. When your weapon has no shots remaining, before you reload, switch to your other weapon. Find an ally that you can switch weapons with, then quickly switch to the weapon that needs to reload and swap with the ally. If done correctly, you will see your ally reload the weapon. Swap your weapon back to see that your clip is now full, and it did not take away any ammunition.
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