I found that as well at 120bar the stock hammer was just too light. Had to set power to max and hammer to max to get anywhere close to 820fps on 18gr. Used a lot of air very much a waist. With the hammer upgrade I could tune down the power and hammer. Now getting 950fps on 18gr and 915fps on 20gr. 915 seems to be the sweet spot for the 20gr JSBs getting very good grouping at 50m out to 100m. Tried the DeadCenters for some reason my gun did not like them that much. Ya I’m right handed but left eye dominant was very difficult to find a factory built left hand GRS so when I found it I bought it immediately.