Spring Loaded Paintball Gun

[RileyJames]

Hello, been reading this forum for quite a time, i really like it, it's a bit overwelming at the beggining but now i feel more confident with spudding hahaha

the reason for this post is that i want to build a paintball gun or rifle, based on the drawings attached, it's way too simple, but i think it will suffice. The problem is that i need to choose a spring and a piston chamber. What do you suggest? The projectiles are .68 regular paintballs, i want at least 150 fps. Also, are there any desing flaws? Is a gravity loader a mistake in this type of gun?
Thanks in advance.

Edit: there's a mistake on stage 2, it's obvious that the spring relaxes hahaha

Great project! Use a strong spring and a well-sealed piston chamber for 150 fps. A gravity loader can work but may jam—tilt or agitate for smoother feeding. Keep testing and refining!