I only know the Nyko Perfect Gun, it's just a plastic shell but I didn't like it. Of course is more "cool" to have a gun-type control instead of just pointing, but it could broke easily.
Sometime ago I was interested in this one
It's not a plastic shell, but literally a Wii Controller in the form of a gun. Looks really good, but have some mixed reviews.
And I just found this one
Has good reviews, and an integrated nunchuck, and looks kick ass.
Castlevania Judgment FC: 1161 - 3389 - 1512
3DS Friend Code: 3480-2746-6289
Wii Friend Code: 4268-9719-1932-3069