This was a quick project that I did while my friend was over who knew a lot about this stuff. It shoots one of those technic L shaped, 4 hole long pieces. It uses the same priniple as the Mini Thriller from Badass Lego Guns book by Martin Hudepohl;  the trigger (when pulled) causes a piece to move out of the way of a brick (ammo) that is being pulled forward by rubber bands attached to the "wings" of the gun. The gun has room for 3 extra bolts/ammo: 2 in a case thing on the stalk and one connected to a loose rod. It is pretty powerful for 1-2 hours of work (can blow apart a lego target-see video 2.) The bolts/ammo can vary but need to have a height of at least 2 pieces.
<p>hi I added this instructable to my collection </p><p><a href="https://www.instructables.com/id/My-Favorite-Working-Lego-Guns/"><font color="#0066cc">https://www.instructables.com/id/My-Favorite-Working-Lego-Guns/</font></a></p>
Awesome crossbow. I was just thinking of making one that was mini and compact would it be ok if i took your design?