I've tested my "arsenal" of Nerf guns, and found that the scope works best with the Maverick and the Barricade guns.
-Cheap Toy Binoculars
-Nerf Rail Clip (I used the flip up sights from the Recon)
-Hot Glue Gun and/or Duck Tape (hot glue strongly commended over duct tape)
Depending on Rail clip and Binoculars:
-Possibly a utility knife or hacksaw
Step 1: Disassemble Binoculars and Rail Clip
I was lucky and found my binocular scope already disassembled from the other part of the binoculars, so you will have to figure out how to unscrew or cut off one of the sides of the binoculars on your own. save the half that doesn't have that middle part attached (thing that regulates focus/holds them together). This obviously will become the scope part.
Flip Up Sight:
Again, doesn't have to be flip up sight, you can use another similar attachment's rail clip if you would like. however, I prefer you use the flip up if you have it since it looks good with the mod, as well as the point that the flip up on its own is completely useless and doesn't work (might as well find a use for its parts). So, either yank off or cut off the black flip up part so that only the yellow body and black rail clip parts remain. That was easy.
Step 2: Glueing (or Taping)!
NOTE: I did this the first time; MAKE SURE YOU DON'T GLUE IT ON BACKWARDS! It's easy to forget really.
Once you get your sense of direction, find a good spot where your monocular? fits relatively well or interlocks. DRENCH the bottom of the scope in hot glue, then stick it directly into the desired place. Once that batch hardens, try to cram some more hot glue into extra holes or concealed areas to guarantee a strong bind.
Step 3: Trial
Since un-modded Nerf gun typically dont shoot horizontally very far, the scope wont be great for all guns, but is a great replacement for guns that use ACOG. So rather than ranting on how this doesn't make you the COD sniper you want to be, be grateful you have a replacement for the sucky stock ACOGs Nerf makes.
Thanks for looking. This is my first instructable, so I appreciate any feedback or questions.
I'll be making an instructable on how to make a Nerf wrist rocket concealable nerf gun mod later.