I wouldn't go more than a 45 degree angle, and even then only when I'm peeking out of a corner.
But I'm interested in seeing a video of someone trying to fire a GBB upside down... try it out.
"War must be, while we defend our lives against a destroyer who would devour all; but I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend: the city of the Men of Numenor, and I would have her loved for her memory, her ancientry, her beauty, and her present wisdom. Not feared, save as men may fear the dignity of a man, old and wise." - J.R.R. Tolkien