I bet a motorcycle or automotive shock absorber could be turned into a wicked spring rifle... might be on the dangerous side tho! Inspired by seeing one in the dumpster the other day.

They built a balanced piston valve. Very heavy-duty steel, they've run it as high as 500psi. The barrel is evacuated before the shot. There's a nice shadowgraph image of the shock wave from the supersonic baseball. Discussion of the concept: https://youtu.be/E60G3ba4TUA?t=296 The shock waves start h...