I have a .22 Kozak (not FC version) 550/290. It shoots the JSB 18gr at about 935fps (35fpe). These were from a Caldwell rest back in May, at 50 yards. I would definitely agree with the suggestion to change pellets.
I don't have pictures, but I have also had very good luck at 50 yards with the JSB 15.89 Hades, and FX 15.9gr, both traveling about 975-980fps. Benjamin Match did not do well at 50. I have a tin of Apolo Monster 25.4gr, which did well enough at about 800fps that I need to test again.
I haven't made any hammer or regulator adjustments to the gun (though the previous owner did), so all these were fired with the same settings, from magazines.