This guy has built himself a replica Halo Assault Rifle that not only looks good as a replica Halo Assault Rifle, it's also a fully-functioning Nerf gun.
Moz, from Halo cosplay community the 405th, took a cheap "dollar store" plastic weapon, a Nerf gun and tore both apart. After a paint job, mechanical tinkering and some added detail to the outside of the rifle, it was good to go. It's even got a torch and an ammo counter.
Nerf Ma5B [405th, thanks Tom!]