It looks like none of you noticed that the gun handle actually IS a nunchuck as well. You see that thing on the outside of the curve? That's the nunchuck joystick.
There must be a missing button or two, though, so it still wouldn't work with a game that required buttons on the nunchuck as well, unless they were doubled up on the controller side.
Upon taking a closer look I also noticed that it looks like you can hold the thing with two hands and have a button for both index fingers: on at the trigger, and one at the wiimote button. Holding the gun that way would also give you access to the buttons on top of the wiimote without getting in the way of anything.
So in summary, this just looks like the nunchuck in a different shape, maybe held in different hands.