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This is a great idea, but few improvements will go a long way towards the easy construction and firing of this device. Please take these points as constructive criticism, since I admire your work and the fact you're sharing it with us.

  1. Enable the nock to go into the tunnel without leaving it. I don't have the proper technical names, so I'm sorry if it's unclear, but the fact that the nock can just leave the tunnel at any timepoint is really annoying, and also detracts from the accuracy.
  2. Create hooks or other similar add-ons on the nock, to tie the bands there. Tying them around the nock itself is particularly bothersome and prone to mishaps.
    I would do the alterations myself, but as I have no skill with 3D designing, all that's left for me to do is give advice to others :)
    Again, thank you for sharing your work!