LEGO builder extraordinare PLUM B has built this life-sized replica of a Gears of War Lancer rifle that doesn't just look the part, it shoots the part as well.
In the video above, you can see that you can slide magazines in. Then cock the rifle. Then actually shoot at stuff. And when the Locusts get too close, you can fire up the chainsaw, which (sort of) works.
Lego Gears of War Lancer Assault Rifle with firing action and motorized saw blade [The Brothers Brick]