[Speedle]

Doesn't such sleeve add more friction compared to two separate narrow glide rings? I mean such sleeve has more contact surface.. Of course standard glide rings suck. Mine BAR10 even scratched teflon cylinder. But decent material glide rings look superior.

 

[Speedle]

Teflon is kinda soft as raw material. Nylon/PA6 isn't the best option for sliding. I used Ertalyte for receiver glide rings and piston glide rings. Now the bolt pull with teflon cylinder is very smooth. SPR380 spring feels like M150.

Regarding the tolerances (though diameter would be a more correct term here) - I left the same diameter as on original rings. Cylinder head acts as a base surface with hop up unit. So some clearance between rings and cylinder is not an issue. That gap even helps to reduce friction.

 

[Speedle]

For high end builds you might also like Iglide/Iglidur bushings from igus.com.
At work we used their bushings in moving components of marine robots instead of ball bearings. Worked pretty well. Friction of those bushings is really low.