Not advice just relating my findings. HW 77 cal. .177 8.3 gn Superdome New seal tight 595 ft/sec
slacker seal but still push fit 740 ft/sec
slack seal piston falls under own weight 750 ft/sec
Got spring data somewhere, for the .22 and .177 will have a look if you're interested.
Both make power (too) easily. Very pleasant at around 11 ft.lb. with Superdomes.