I would think if you see 15fps spread with weighed pellets you should be sending it back anyway! 12fps unweighed is what you should be looking for and less with weighed ones.
A low shot on one of the AA rifles, EV FTP etc if it is the reg would indicate a sealing problem so the reg is over pressurising the valve chamber, this results in the valve not being opened as far when the hammer strikes it due to the higher pressure behind the valve resulting in a lower power shot. If the reg pressure is creeping up a longer delay between shots would cause a low shot while repeated shots in quick succession would see them all pretty much ok so try that to see if it displays that tendency both at targets and over a chrono.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.