Originally Posted by Big Dave
I've not noticed any pressure loss from the cylinder but I will check it overnight.
The drop in POI is repeatable and will drop further if left for longer.
If you have a leaky reg or firing valve you should be losing air from the cylinder as when the reg pressure drops too low it will refill from the cylinder. If you aren't losing air at all then it could suggest something in the action, have you tried chronographing that first shot? if the power is low try repeating the exercise in a warm house.
It's also worth doing a string of shots over the chrono, fill the gun up then fire 10 shots, wait 5mins & fire another 5, wait 10mins & fire another 5, write all the results down & see if a pattern appears