I liked the original but had a hard with it not shooting where I wanted it to and not being strong enough to shoot very far. So I created this redesign. The bar should fit in the existing base hole where the rubber band used to fit. Simply print it out and slide in place. Wrap a rubber band around the top cross member and hook both ends in the slot on the back of the improved arm.
This should shoot significantly further and straighter.
Extra Bonus you should be able to use your catapult without the extra feet that some people have printed.
Just print then plug and play. No raft or support is needed to print this out.
I recommend you print the arm at 50% infill or greater because it is going to be under more stress than the original.