I came across this awesome original tiny crossbow design and loved the idea, it's very impressive to me that 3Dennis designed a functional crossbow using filament as the string. The documentation and instructions are top notch as well.

However, I ran into a number of mission critical issues when attempting to play with it:
- Bolts: loading the bolts was difficult. There is very little helping you hold the bolt in place while you attempt to set the string, and even more importantly I could not get the bolt to ‘hook on’ to the center piece of the body. I could only use the crossbow by manually pulling and releasing the string, and this was true across multiple materials and print profiles.
- The support for the lath (i.e. the secondary ‘mini’ bow portion) repeatedly came out malformed because it barely touches the plate, making the slicer not put down a first layer for too much of it.

I set out to make some improvements, fulfilling the concept of loading a bolt, setting the string, and pulling the trigger at will. A number of small changes have made it much more reliable, and much more fun!

The bolts have been adjusted to have a much more pronounced and shaped rear hook and significantly more effective string groove, such that the force of the string properly holds the bolt in place. The groove is positioned slightly above the centerline of the bolt, making the tension go forward and (critically) slightly down. The right and left body pieces had their height raised which helps with handling and loading bolts, and a small hook on the front of the bolt helps as well.

The support lath had some material added to the tips so we're not printing in the air. The toy lasts longer now because that part is shaped properly. Lastly, the trigger has been reshaped to match the new bolt shape and it does a much better job of functioning appropriately.

I changed up the card to help differentiate it from the original, and there's an extra bolt card on a second plate. A cardless profile is also provided.
 

Instructions are identical to the original. PLA will work but it's very stiff and it won't last that long, ABS is great and PETG is good too. It actually shoots the bolt with quite a lot of force, so be careful around people and pets! Happy hunting!