A friend of mine went to a local machine gun shoot recently, and came back with some footage of a pair of Bren guns being fired with a 100-round drum magazine. These drums were designed for anti-aircraft use, and are quite rare today, so it was cool to see one actually in use. You can see a bunch of photos of the drum mount and the drum itself (including a photo of one disassembled) at: http://www.forgottenweapons.com/bren-100-round-drum/