The nerf Dodeca-dart is a 12-dart grenade or buckshot. This can serve as a magnum round in a homemade nerf shotgun. In the pic, it is a grenade.
Step 1: You Will Need
12 elite or streamline darts
2 rubber bands that fit snugly but not too snug
A key ring or paper clip
Step 2: Grenade
Bundle them up and slip a rubber band on the tips so they fan out. Put the second rubber band with the pin on the bottom to hold them together and to prevent premature detonation of the grenade.
Note that they naturally shape themselves into a triangular configuration so this may not work in gas powered grenade launchers.
Step 3: Buckshot
Omit the bands and put them in your shotgun so they spread when fired. Also note that a loose rubber band can be placed on the bottom for storage as this will fly off when fired.