This is something I've been working on. It is a small, pocket sized crossbow that can shoot very small projectiles. Just slip a small string or thread (dental floss also works well) through the hole on the end of the rod, and tie it to the rings on both arms. If you have feedback about my design, I would love to hear it!
I'm working on designing a bracelet sort of thing that connects to this so you could fire this from your wrist, so that should be out within time.
Here's my design for ammo to this crossbow: https://www.thingiverse.com/thing:3494384