Funny how when you make a rubber band shooter, you suddenly have no rubber bands. This shooter will certainly make them go missing in a hurry.

Unlike most styles of rubber band shooters (paddle wheels, rack shifters, etc.) the radial array of bands allows for extremely high density while also maintaining a constant band tension and trigger force. The removable magazine can hold up to 21 bands and can be swapped out quickly for heated rubber band battles.

The mechanism uses a worm and gear for both advancing the magazine and for releasing each band. Pulling the trigger causes an internal rack to engage gearing that rotates the worm ¼ turn. The worm indexes the magazine while the leading edge of the worm thread pushes the next band off its little hook. The worm has 4 threads, so each ¼ turn advances the magazine by one tooth.

The trigger itself is another interesting mechanism. When the trigger is pulled, the little blue cam (hiding behind the rack in the 6th photo) pushes the rack up to engage the lower gear. Continuing to pull the trigger advances the gear by ¼ turn. But when you release the trigger, the cam flops down and lowers the rack as the trigger returns to keep the rack from reversing the gear.

You can shoot in semi-automatic mode (one shot per trigger pull), or you can create a rapid-fire burst mode by rolling your palm down along the worm. As a bonus, the burst mode makes the magazine self-ejecting!

Oh, and I almost forgot – I've also included a little stand for holding up playing cards as targets.

Note: This model require a well-tuned printer. A number of people have had trouble getting this to work, and besides the usual problems of over-extrusion, blobs, etc., this model requires your axes to be perfectly square. Measuring squareness is difficult, and adjusting it on most printers is even more difficult. On my own Prusa Mini+, I had to add a shim to square up the X and Y axes.

Printing
I highly recommend printing this in PLA. There are a couple of internal leaf springs, and there's no guarantee they'll work properly if printed in PETG , ASA, etc.. I recommend 0.2mm layer thickness. Print one of each part. No supports are needed, but note that the rack has a small built-in cylindrical support hiding in the little pocket where the cam goes. Remove that support with needle nose pliers after printing. Test fit all the parts before doing the final assembly. Run all the moving parts back & forth against each other until every thing moves smoothly.

Assembly

Insert the cam into the pocket in the back of the rack as shown. You'll need a short stub of 1.75mm filament to act as an axle. Insert the filament stub through the hole and cut off flush on either side. Then push it back out partway and smash the exposed end slightly with pliers and press it back flush. The deformed end will hold it in place.	Insert the rack on the little post on the trigger. You'll need to tilt the rack down somewhat to snake the little cam spring behind the leaf spring on the trigger. At this point, make sure that the cam pivots freely and springs nicely to the extended position. Also make sure that the rack pivots freely and can be pushed downward by the leaf spring.
Place a little oil or light grease in the trigger slot in the body, and then insert the trigger assembly as shown. It should slide back & forth easily.	Place a tiny dab of oil or light grease on the two shafts in the back. Install the worm, followed by the gear. Both should spin freely.
You'll need a small rubber band to act as a trigger return spring. Install it as shown.	With the trigger mechanism, gears and rubber band installed, you can now snap the cover into place, and you are done!
If you should have to take the cover back off, note that there are little slots located around the perimeter at each snap point. Just insert a small flat bladed screwdriver into the slot to pry the cover open.	Lastly, test fit the magazine on the shooter. Rotate the worm by hand to engage the teeth in the magazine. The should be able to rotate with very little friction throughout the entire range of the magazine. Work the worm back and forth over any rough spots. If you like, you can add a little light grease to the worm, but this can get messy as the worm is exposed.

Operation
First, you'll need to load your 21 rubber bands onto the magazine. Start from the far end as shown in the 4th photo at the top of the page. (The video above also show how to load the bands.) Insert the post at the nose of the shooter through the keyhole slot in the tip of the magazine and then rotate the magazine clockwise until it presses up against the side of the worm. Pull the trigger to rotate the worm ¼ turn (or just rotate it by hand) so that the first thread engages the first tooth of the magazine. Pulling the trigger with advance the magazine and also release the first rubber band. (You'll notice the rack pops out the back end of the body when you pull the trigger.) Keep shooting until the magazine is empty, and a couple more pulls of the trigger will release the magazine. Alternately, you can roll your palm or thumb down across the top of the worm to create rapid fire bursts.

Which Rubber Bands?
This can be used with either 1/16" (nom.) wide bands or 1/8" (nom.) wide bands. (1.5 - 3mm). The thinner bands are easier to load, but pack less of a punch. The ideal length of band (usually measured as ½ the circumference, or the length when smashed flat) is about 2" (50mm). Rubber bands will stretch, at most, about 3 ½ times their length, and for this shooter, they need to stretch about 6". 2 ½" bands work pretty well too, and 1 ¾" bands would be the absolute shortest you could go. Keep this in mind as you head to the office supply store.

Overall Size 14 rubber bands (1/16" x 2") are ideal, but are harder to find. Size 16 (1/8 x 2 ½" work well too, and are stocked in most office supply stores.