This is a speed loader for the Nerf Rival Kronos. Similar to other speed loaders it holds 5 balls and allows quick loading of them all at once. This model has a sliding pusher to grab onto instead of having to use your fingers to touch the rounds directly. This design gives two major advantages:
1) It's double sided, load the speed loader itself from either end. Easy load by just pressing the speed loader directly onto rounds sitting on any surface.
2) The sliding pusher extends past the end of the load chamber, thus forcing the balls far enough down into the Kronos to line up properly. Most other speed loaders you have to load the blaster, remove the speed loader, then use your finger to push the rounds a bit further in, then close up the blaster.

I'll probably refine this a bit more by making the sliding pusher tighter to the chamber shaft, and probably curve the rail slots at the top and bottom for slight nicer printing, but this model works great as is.

No need for supports, it has the only ones you need built right into the model, just snap them off when it's don't printing. Highly recommend you use a decently wide brim when slicing.