Considering the *striker mechanism and the heavy ex valve spring, I am thinking the lift and duration of the ex valve is independant of the realitivly low reservoir pressure used in this rifle. Therefore I would think that the lift and duration would be same at lowest reservoir pressure as at high reservoir pressure.
That leaves a huge "lost volume" transfer passage to possibly pressure up (at lowest reservoir pressure) before the shot ball even moves. The ball would stay put owing to it's own inertia, static friction and possibly a bit of intentional restriction at the breech to prevent the shot ball from rolling out of the barrel since a patch was not used.
So when viewing the plan view of the Girandoni, could it be that the huge transfer passage we see was actually necessary/required for the proper function of this rifle?
* I am aware that there is a differance of opinions on how the striker mechanism functions. I am assuming that the striker disengages from the ex valve and with a stout ex valve spring, the lift and duration remain constant throughout the gambit of reservoir pressure variations.