I found a good polish of the internals on my HW99 .22 and the tinbum kit worked wonders.
I had to cut 2 or 3 coils off the spring to bring it back under 12ft lb. After bedding in.
Now it shoots at about 11.2 and is very smooth.
Just give the rearward parts of the piston a polish and the ends of the spring and as much of the inside of the tube as you can reach.
Knock off any rough edges around the cocking slot and remove any factory swarf or filings.
I bought some 0.5mm delrin from and made a piston sleeve.
Do not go too mad, as you need the surfaces to hold on to some lubricant. I lubed mine with moly grease.
To polish the ends of the sping I taped some very fine wet and dry paper to a flat bit of glass and ground it nice and smooth.
The Tinbum Kit and a bit of gentle elbow grease, is all you need, no need to spend more.
Ouch!
Ouch!
Barn Door Scarer
HW95k .22, HW50s .22, BSA Goldstar SE .177, AA 510 Ult Sporter .177