Quote Originally Posted by piggy589 View Post
Cocking the gun with the loading tap open and then closing it produces a hiss as air is drawn into the vacuum created.

Suggesting to me the new replacement piston seal is doing its job.

As you say the swept volume alone should produce over 11ft/lbs.

Hell,I've got a couple of sixties Meteors,.22 and .177, both nudging 11.
The seal might be good but one Mercury I had was the same, a good seal but the friction of the O ring was slowing the piston down a great deal. The other thing is that sometimes they can be over-sprung, causing piston bounce which reduces power. It's a conundrum to be sure. The game is not helped by the unpleasantness of stripping them and building them, always found them the most difficult to re-spring due to the odd shaped back block and the amount of preload they have.