I made a cardboard crossbow before this, but it eventually broke. I decided to make a new, better, more durable working cardboard crossbow. How it works is to pull back the drawstring, open the trigger, put the string in, close the trigger, and put the projectile in after you release the string.
Spooky Scary Cardboard Crossbow
A cardboard crossbow that can shoot pencils(and other things!) and the power of the shot depends all on the trigger and drawstring.