Purchased the piston seal replacement for the Hatsan MOD 125th from Custom Airseals out of Australia after first replacing the the breech seal. According to the email I received from Custom Air Seals the piston seal was supposed to measure 29.20mm +/- .5 . See results below:
- Hatsan Stock Piston Seal: 28.65
- Custom Airseal Claimed (smallest): 29.2 -.5 = 28.7
- Custom Airseals Actual: 27.87
Result: Zero compression to push the pellet out of the gun.
Maybe look at the Air Venturi Piston Seals.