Our tuning rings for Bb clarinet are now made from solid brass and sized at 0.5mm, .75mm and 1.0mm. We have found that solid brass rings provide increased resonance and projection.
Why use a tuning ring?
When you pull out the barrel or the middle joint you create a gap that acts like an enlargement in the bore, which always affects the pitch adversely.
In the case of Buffet clarinets, the pitch of throat E and F drop significantly when that gap is present, more so than the tones above and below. Also, most manufacturers make barrel joint sockets with a gap of anywhere from .005" - .010" between the barrel joint and the top joint of the clarinet. Adding a ring will reduce this descrepency. I find that when there is larger than a .020" gap (.5mm) there is a significant loss of energy. Having a ring helps reduce that loss.
If you pull at the middle joint (as I do) a .5mm ring between the joints is a great idea. It greatly improves the response and keeps the low C from dropping in pitch too much and also keeps the top of the staff "F" from going too low.
We make all of our tuning rings from solid brass. Grenadilla rings are too fragile and I do not like the sound of hard rubber or plastic rings.