Good post Kieran, thanks
I would like to note the difference that different pellets can give to the power, even if they are the same weight.
The other weekend one of my mates bought a new pcp rifle that had a manufacturers test card with it saying that it was shooting 11.5 ft lb.
He put it over the chronograph and it was shooting 10.2 ft lbs so said he was going to have to send it back to the manufacturer to fix
He was using a pellet brand that uses other Metals aswell as lead in the manufacturing process that make them quite polished and almost chrome in appearance.
I asked whether he had used air arms as they have a very high lead content and are softer, the pellets were the same grain/weight.
He didn't have any but i did so he used them and he ran it over the chronograph - 11.5ft lb
The manufacture test card, it says 11.5 ft lb using Air arms
I believe this is down to the tighter seal that the softer lead that air arms use creates when the blast of air occurs.
I have found a similar effect when sizing pellets.
It shows what a challenge the manufacturers have when setting the power on on pcp's.
And why your post is good as it is right to challenge our misconceptions sometimes as it too easy to blame the rifle itself, not our testing procedure.