This is a auto-loading gun that is very simple.
It dose not use a trigger but you pull the pin back and let go to fire.
Step 1: Barrel
Build this (I couldn't find my whites so I used 3 yellows instead of whites)
Step 2: Auto Loader
build this. This is a bit tricky
Then put a white rod so that figure 5 is symmetrical.
Step 3: What You Have.
Build the firing pin. you should have these parts.
Step 4: The Gun
put the clip on like this
Step 5: The End
Put the elastic on like this
To shoot put marbles in the top like this then pull pin back.