It was quite clear from what Jim said what was needed,
If you're absolutely sure about the specification you need
which obviously means a particular wire thickness, number of coils, length etc ... but then everyone starts firing off contradictory stuff. I'm surprised he bothers posting since NO ONE LISTENS TO HIM.
Is this part of the corrosive thing whereby opinions are the same as facts and we disregard experts because my opinion is as valid as anyone else's therefore we are all experts swimming in a sea of alternative facts?
If you want a nice smooth action, just cut some bath sponge and stuff it in behind the piston and the back block in leiu of a spring. Beautiful smooth almost recoilless action. Pellet stays where it is, but you know, saves money on ammo.