Kinetic here with my most recent gun, the KSP (Kinetic Sling Pistol).
My second entry in the K'Nex Innovators' Contest, this concept really pushed my limits as both a builder and as a designer, mostly because it is nearly the exact opposite of my building style. I always have found it easier to build bigger guns because, no matter how complex the design, you almost will always have room to incorporate every necessary mechanism. This gun was very different. Because it is a pistol, it needs to be small, and that alone was a new concept for me. But the fact that it was a sling pistol required some complex mechanisms for it to function (at least, the way I wanted it to), and incorporating those mechanisms in such a small space was quite a challenge.
I consider this the first TRUE sling repeater, because although we have seen those in the past, they either require you to manually pull back the rubber band, or will simply slide the band past the bullet, which requires little ingenuity and does not separate the shot from the rest of the ammo. This gun utilizes a slide (which also acts as the upper rail, my favorite part of the design), as well as a bullet transfer mechanism that separates the loaded shot from the unloaded shots. You never have to touch the rubber band, nor does it slide past the bullet.
This was by far the most challenging gun I have ever made, but I think that it will prove a key aspect in my future designs. Although reliability can be a factor (bullets sometimes jam), I am proud of this gun, and consider it a worthy entry for round two of the K'nex Innovators' Contest.