Ok well lets see here. The Guns look like this.
(https://www.kumoricon.org/forums/proxy.php?request=http%3A%2F%2Fi365.photobucket.com%2Falbums%2Foo97%2FHaou_Judai%2Fhguv3-haseo-xth-form-weapons.jpg&hash=90dde513880b45827ebb4a524b8a8347a6555cce)
(https://www.kumoricon.org/forums/proxy.php?request=http%3A%2F%2Fimages.cosplay.com%2Fphotos%2F22%2F2213377.jpg&hash=17feeabc82d2955d6ba4e52c02c61a5a42745aa0)
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=OrWGAyA7jQw
Use 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
(https://www.kumoricon.org/forums/proxy.php?request=http%3A%2F%2Fi484.photobucket.com%2Falbums%2Frr202%2Fneko_photos%2FHaseocosplayThing.jpg&hash=0b065ea0a2b693f0bce2f5efa8276ea1f98b5571)
The images I used as a reference were...
(https://www.kumoricon.org/forums/proxy.php?request=http%3A%2F%2Fwww.swisslink.com%2FFS%2F1%2FAtom12234.jpg&hash=5fce20bd953b9d58fa7dfc3160d4f3ef3ca3e18f)
(https://www.kumoricon.org/forums/proxy.php?request=http%3A%2F%2Fdroidz.org%2Fshare%2F22823%2F1245243460.gif&hash=fadabb6ddbc66bf8c3a153de75d08a5b889bd17d)
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.