It only charges to 800 psi with its modest sized air tank, yet claims can shoot 40 rounds before losing much muzzle velocity which is comparable to .45 acp. In comparison a modern .45 air rifles that charges to 3k psi and shoot much slower (650 fps) can only manage 6 or so full power shots.
I would suspect that the Girandoni Air Rifle would probably get 5-6 useful shots of a 140 grain ball at around 400-500fps perhaps 70-100 ftlbs (enough to quite lethal from 0-100 yards) As the PCP chamber bottle was essentially unregulated there would be a good deal of variance of the exit muzzle velocity and consequently the ballistic drop would drop and drop. The carry of 2 additional bottles would cover the 20 round magazine. The rifle being the very first repeater would have been quite a rapid fire rifle compared to the much more powerful muskets of the day. Accuracy would probably have been quite good out to 100 yards when drop compensation is taking into account, hence the special training required for the rifle.
As a survival hunting rifle (where conventional ammunition is very short supply) I suspect it would be useful even today considering the 240 year old design. (100 years before smokeless propellants)
More modern rifles would be preferred though such as the Daystate Wolverine being exceptionally accurate.
http://www.youtube.com/watch?v=Ysvz378Ttjw
)