I have ran out of Cardstock, so until i get some more i have been working on the idea of using the Nerf Rayven CS-18 as a base platform for my Halo 4 BR build. So here is what i have come up with so far.
I think NERF guns are the best base for making weapons. But choosing the right one is the prob. If I remember correctly the rayven has a grip much like the AR. Are you planning to trim the grip to make it look more like a BR? I personally would use recon for guns like the BR. Your choice. Good luck!