Uhltimate's Disc Shooter is an exceedingly cool bit of engineering. It uses a springy fork to launch a disk forward. The disc starts out in the center of the curved fork prongs, and when you pull the trigger, the fork is pulled back around the disc, forcing it through the springy prongs and launching it forward. The original model page has great details on how it works.

Ergonomics: One drawback of the original, though, is that it takes a bit of trigger force to launch a disc, and it can get a little uncomfortable pulling on the trigger. So instead of pulling the fork back around the disc with an index finger trigger, this design pushes the disc forward through the fork with a thumb actuated trigger from behind, which is much easier to do. The resulting design is odd, but comfortable to hold and use.

Performance: After completing my design, the first thing I noticed, rather unexpectedly, was that the discs flew a noticeably faster and farther than the original. Analyzing video of both versions, Uhltimate's design had a disc velocity of 20m/s, whereas my design came in at 28m/s -- a 40% increase. I also measured flight distances for mine of up to 15m.

I think the biggest factor might be in different dynamics of the two versions. In the original, as the disc is being accelerated forward, the reaction force is pushing back on a movable fork, which has relatively little mass. So some percentage of the energy is going into accelerating the fork backwards. In my design, however, the fork is rigidly attached to the shooter body (and your hand) which are relatively massive, and hence, very little energy is lost in the recoil. (It's a conservation of momentum/energy thing.)

The other factor I can think of is that in the original design, during the acceleration phase, the disc has a whole stack of other discs being pressed down on it, creating a bit of drag. In my design, the disc has mostly exited the shooter during the acceleration phase, allowing it to be launched unencumbered.

Aesthetics: Well, the original looks like some sort of retro sci-fi weapon, where as mine looks like a water tower with a handle. I tried to emulate some of Uhltimate's design cues, but no doubt, the original is cooler looking.

Update: After researching disc shooters a little more, I wanted to give credit to a much earlier design from davedelay on Thingiverse that is much closer to this design. Please leave a comment if you know of an even earlier version.

Printing

All parts should be printed in PLA. I used 0.2mm layers. The springy fork actually prints-in-place between the handle and the disc silo. The springy fork layers (from 57mm up to 61mm) should be printed with 6 perimeter lines to insure that the fork prints solid. You can print the entire model with 6 perimeters, but you can save time and filament by setting a Height Range Modifier in PrusaSlicer for just that range. Lastly, the body should be printed with a brim. The 3MF file I've provided has this all set up for you.

Assembly

After printing, the prongs of the fork will be lightly adhered to the layers above and below the fork. You may be able to just press on the tips of the prongs to break them free, but you can also shove a razor blade or x-acto knife into the gaps at the tips to break them free.

To assemble the arm, as shown in the 8th image, you will need a short piece of filament to use as a hinge pin. It should press lightly into place. You will also need a short (~25mm), thin (1.5mm) rubber band to loop over the back of the silo and into the notch in the back of the arm. If you don't have a really short rubber band, you can tie a knot in a longer one and cut it off, as shown in the last photo.

To assemble the plunger, loop another short rubber band over the tip of the plunger and stretch it onto the gap on the underside. (See 9th and 10th images.) Start to insert the plunger into the back of the body, but before the free end of the rubber band disappears, loop it over the tiny hook at the bottom of the plunger slot as shown in the second to last image. The plunger should operate freely and spring back open all the way.

Launching

Use your thumb to pull back the arm and load up to 8 of the disc into the silo. Remember that the discs launch in excess of 60 mph, so be very careful where your point the shooter for your first test. It's very tempting to look at the end of the shooter to see how the disc comes out – resist this urge! And as usual, do not shoot at animals, pets, siblings, or anyone or anything that could be damaged by a 60+mph projectile.

And please, do not use this outdoors. You will lose discs. A lot of discs. And we don't need any more plastic pollution.