The shot capacity would appear to be a little disappointing, however scrutiny of your muzzle velocities seem to indicate that the gun has not been filled to a high enough pressure.
To pressurise a gun to the "heart of the fill" and maximise the natural sweet spot, the velocity should rise slightly, then fall, I would suggest a fill pressure of at least 220BAR after looking at your results.
It would be worth while experimenting if the gun can be safely filled to higher pressures.
David