My Hatsan is comparable to a new Hatsan 155 with an OEM spring that degrades overtime. When I decided to convert it, I was looking at a muzzle velocity of about 830 - 850 fps but was very amazed it surpassed my expectation. The recoil is sharp but NOT harsh, unlike the OEM Hatsan. The next thing I need to do is to replace the piston seal or convert the parachute seal to an O-ring ... will be my next project.
Recoil and noise are very much subdued. Accuracy has improved very much. Able to hit a target with 5 shots at 50 yards .75 inch ctc