This is a 2 disk magazine for the Nerf Vortex Proton disk gun. That is ordinarily a single disk gun so these allows for 3 shots (2 in the hopper and one loaded) between loading.
You can see it jam a little in the video which seems to be caused by the safety systems Nerf builds into these things. It acts up about the same without the hopper attached. The next step would be to remove these safety mechanisms which is covered by other people online. Larger clips can be generated from the scad file if you desire.
I've added a 5 disk version which I have not had time to test fit. I've never printed that particular file.
All of the trademark terms are property of Hasbro and this is not affiliated with them
The pullback is screwed into the holder across the old finger hole with a #4 sheet metal screw or similar. The hopper is attached with a drop of glue. Use removable glue (hot glue) if you intend to take the gun apart later. To make everything line up to drop in nicely I needed to use a glue with a bit of volume to tweak positioning.
Screw the pullback in tightly and keep it to the back of the space it will fit in. There is enough slop in the positioning that the pullback can hit the release tab on the back which will jam the gun.