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.
Step 2: Lower Barrel
This is the lower barrel. This supports the upper barrel as well as being the attachment point for the upper barrel to the handle and for the rubber bands. 1) Build. Also, build its clone, minus the white rods. 2) Attach as shown.
Step 3: Handle
Build handle. 1)Side view. 2)Top view.
Step 4: Trigger
This is the trigger. 1)Build.
Step 5: Assembly
1)Barrel. Attach as shown. 2) Trigger. Attach as shown. 3. Handle. Attach as shown.
Step 6: Rubber Bands.
I would have notes, but I can't figure out how to add them. If someone could tell me how, it would greatly help with the clarification of this step. Anyway, you thread the rubber bands through the first slot between the upper and lower barrels from each side, attaching them to the first white rod between the upper and lower barrels. This creates a very strong connection, as well as making it so that the rubber bands won't fly off when you fire. NOTE: picture shows rubber band hooked over a white rod so that it goes over the barrel like that; this was just to store it so it didn't flop around too much when I was taking the pics. NOTE 2: it is MUCH easier to partially disassemble the bottom half of the barrel to add the rubber bands. trust me, i've build several of these things.