Yes, I know its function, but it's sliding upwards after every shot and hence is losing FPS over the chrono. I can over-pressurize to 300/85 and shoot 150FPS.
The cap is just there to prevent the bolt from sliding out, as far as I know it has no immediate effect on the gun's performance.
Try changing the 006 o rings to 007. Those two at the bottom part of the engine.