I can't recommend to make this rubber band gun! It's a lot of work to make this gun and it's not worth it. It's only a prototype. It's not reliable.
A weak rubber band will stop the mechanism. The mechanism needs ca. 20% of the stretch power of each rubberband. So it's underpowered. It's difficult to get the mechanism to work. I don't have much fun to play with this gun because of all it's disadvantages.