I used a lens from a broken Lomography fisheye 35mm camera and built a housing for it so that it can attach to an iphone SE, iphone 5, or iphone 5s. These Lomography fisheye cameras can be found cheaply on ebay and I imagine it would work with some other lenses too. It has a detachable bracket to attach to a Nerf gun. It comes in 3 pieces: the phone backing, phone retainer plate, and lens retainer. I'm also planning on uploading another file without the lens attachment so you can just use the iphone without the lens.
Use 8 M2 screws and nuts to bolt together and just slide onto the Nerf gun rail for some epic battle footage!