I saw this one the other day, and I did not want to say anything.

Looks like a wind-up catapult. Trebuchets are better, but the increased acceleration could burst the blood vessels in your eyes, before you leave the sling. Don't try this at home.
https://www.youtube.com/watch?v=mo_oigyFlTA
The acceleration is so great, that it can extinguish a burning piano, before it leaves the sling:
https://www.youtube.com/watch?v=6Rn8jBzsg9U
I built a small one, they are very accurate, once you get the sling length/pin release worked out, counterweight and projectile weight dialed in. I can hit a monitor with a marble 4 cubes away, 10 out of 10 tries. GTFO with your nerf guns, I have a siege weapon...