Nice shooting. I had a 54 back in the late 80s and 90s. I loved it...ingenious design. Albeit its accuracy was limited by the lousy RWS pellets available at that time. Even the Beeman pellets were so so. Wish they had JSBs back then.
As far as the scope mounting. This rifle is in essence a double recoil...although it is recoilless. The scope is subjected to forces that are quite strong back and forth and requires a really well built scope, I had a TASCO made in Japan TargetMaster on it without any issues. But used a scope-stop that was part of the scope mount. On the top of the receiver there should be a hole where the scope-top-screw anchors the mount from moving.