There is no hard and fast rule that can be applied to all PCP's, it comes down to the exhaust valve duration of your particular rifle. For example, if your exhaust valve is still open when the pellet leaves the barrel you would see a big benefit in power or shot count from using a longer barrel, or if your valve is closed long before the pellet left then you may even see a reduction in power and shot count from a longer barrel, i have witnessed both scenarios. The likely hood of seeing a significant shot count increase changing from a 16" to a 20" barrelled .22 at 12 ft/lb is slim.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.