I was trying to come up with a mechanism with a firing pin and a true trigger. This gun is the result and although it isn't very powerful, i'm hoping someone will be able to improve on my design.
Step 1: What You Need
Step 2: Make the Barrels
Step 3: Make the Trigger
Step 4: Connecting Trigger to Barrels
Make two of the first picture
Step 5: Making and Attaching the Handle
Step 6: Inserting the Ram and Adding Rubber Bands
And you're done. Please feel free to ask questions and leave comments.
And remember that this is a prototype and does not work very well. so please, not too much critisism. Thankyou