Red Barrels

Explode all the red barrels on the rooftop while in a helicopter

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On Act 5, Chapter 3, nearing the level you will find yourself in a Helicopter on the gunner, scattered across the building rooftops, there are a bunch of red barrels, your goal is to shoot and explode all of them, this is best to do on the easiest difficulty in chapter select.

This is fairly difficult, especially on higher difficulties.