Watching the slo-mo, seems there isn't enough push from the rubber bands. I see several not staying in the target. Maybe more bands and a longer pull would help make it stronger.
But, I have to admit, there was a lot of thought, imagination and engineering that went into this. Very cool.
On the other end of the road I live on there is a guy who manufactures several items, the biggest seller is the train whistle like you'd see at places like Tweetsie RR. One of the coolest I have seen come out of there was a rubber band machine gun.
Oh, and he is also a licensed automatic gun manufacturer.