This my custom build I designed for the Nerf Platypus: Shell Fed Nerf Rival Kit by RedCowl. All credit goes to them for their original design they created. The only design changes I made on the kit here was the addition of iron sights to the latch and barrel. Also changed the boxy shape of the barrel into a more rounded off design, and created flypoint shells that could hold 7.5mm gel balls.

The original Flypoint shells for NERF darts and the original build guide are included in the files.

Update: The gelball shells work and fire decently with the stock rival knockout spring. I do not own a chronograph so I cannot provide muzzle velocity of the gel balls in the gel ball shells. With 2-3 gelballs in each shell it fires hard enough to be a great shotgun pistol close range.