I actually chrono'd a yellow band (surgical tubing) wrist rocket about a month ago with .68 paint (Valken Echo). I couldn't break 200fps I think the best I got was 197. If you can get one over 300 please post a video I wanna see that rig. You might get there with an off the shelf sling shot & .50cal paint. I've dropped birds in flight with a slingshot before so I wasn't holding it wrong or anything
Somebody asked to borrowed mine to play a game or two with it. I was reffing so I let them.