For ammo I prefer to use an orange connector attached to a blue rod, but you can use anything you can hook onto the rubber band at the back that fits ...
This is my armless crossbow. It uses a common rail design found on many Instructables on this site. The trigger may or may not be original; it's been so long since I made this that I don't remember. If it is yours, please comment and I'll give you credit.
Step 1: Upper Barrel
Here you will be building the barrel. It can be as long as you like; just remember to add one rubber band to the chain for every four segments of body. I have tested this number and it is the most accurate way to get a good amount of power from this gun. The barrel shown here uses a chain of three rubber bands. Build what you see in the pictures. Build only one.