Knex mini cross bow is the result of a few hours of work. It's pretty small, made from knex minis, and I think that it has a true trigger but I am not sure what's considered a true trigger.
Step 1: The Handle
Here is the handle.
Step 2: The Cross Section
Here is how to build the cross section. You will need a thin rubberband long enough to fit loosely around the end (as seen in pic).
Step 3: Assembly
Connect the handle to the cross section.
Step 4: The Trigger
This is the trigger (revised).
Step 5: The Arrow
A blue rod with a purple connector at one end and a yellow connector at the other.
Step 6: Loading and Firing
Insert arrow between guides. Put rubberband in notch on back of arrow. Pull trigger and insert the butt of the arrow under the purple connector. Continue pulling back till the butt of the arrow is completely behind the purple connector.
Pull trigger to fire.