Ok well lets see here. The Guns look like this.
For the blade part I would get like colored plexiglass and cut out the shape then paint on it, glue it to the rest of the gun.
For the gun I would look and see if there are replicas out there that you could buy but they you wouldn't need to make the blade up above.
You could go buy a plastic gun and modify it like they did in this tutorial for Train Heartnet's gun from Black Cat
http://www.youtube.com/watch?v=OrWGAyA7jQwUse that as a basic idea on how to construct it (you know board paper and paint?)
I Grabed some images online and complied them with my photoshop and flash skillz and made this little .jpg
The images I used as a reference were...
I had to screen shot the gif because of its little animation thing
So thats what I provide I'm not doing it all gosh >.> and why would I do this? to put simply I was bored ._. Have fun.