2 sizes of washers are needed and can be Delrin/Brass/Steel of various thicknesses and can be fitted in multiples, to fine tune a gun.
1 Inside the piston, so infront of the tophat on the piston rod (Smaller internal diameter)
2 Behind the spring on the spring guide (Larger internal diameter)
No 1 will add more weight to the piston if you go for brass/steel so tends to slightly dampen the recoil.
Both will cause the spring to compress more, which could lead to the rear of the tophat meeting with the front of the spring guide and you not being able to get the gun to cock, worse case.
As you are fine tuning it will be trial and error, generally one 1mm washer will give around 10/15 fps.
Not sure if there still available but i got a selection of both sizes in various thicknesses from Dave Hall.
Or you can use steel washers from a hardware supplier and polish them yourself.
Average Spring Gun Shooter