Instructions
Enter a custom local game with "Forge" mode turned on. You will need the editing capabilities of Forge to get out of the map.
Run to the northeast corner of the map. This area houses an aquarium full of "Flood."
Place a turret machine gun against the glass of the aquarium where two panes intersect. The turret should face away from the glass.
Man the turret. If placed correctly, most of your body should be outside of the glass.
Hit "RB" to launch into monitoring mode, fly down and to the right, and bounce back into human mode. You will have to do this quickly and it may take a few tries. Either you will end up outside of the map, or you will experience an embarrassing death.