converting from .22 to .177 set up problems

Hi Guys,

just joined the forum and am after some assistance in setting up on .177 which is giving a few problems, on .22 there are no problems whatsoever, but simply swapping to the .177 set up is not working out.
i have set the barrel arrangement up with an overall free length of spacer then spring then hammer on the barrel before fitting to be the same as that of the .22 set up of 210mm and am aiming for 800fps with 8gn pellets (UK 12ftlb limit) a new hammer has been made to fit the smaller external diameter of the new barrel with a weight of 20gm which gives around 250fps on its own, adding two 8gm washers gives 500fps being aware that more added weights also increases spring tension, additional washers increased to just get over 600fps and then we dump all the air in one go…oops, not once but twice..just to make sure and for some strange reason chrono reads in excess of 1000fps.

i would not have thought a 46gm hammer would have done that, but that said there is a large gap behind the top hat and bottle valve of 4mm the one fitted is not adjustable and the bottle has an o ring fitted so looks like this may be a modded valve.

any body any ideas as to what weight hammer in .177 does the job and what assembled free length of hammer spring and spacers may work best.

many thanks, Keith