Hi Thirdhorn,
I can not advice but offer some experience instead. After 32 years of tuba playing (PT and Klier Models 32-33 mm rim diameter), I switched to french horn in 2012. It took me almost 1 year to get accustomed to the much smaller mouthpiece rim of the french horn (I played 17,5 mm). The transfer was very successful. I played horn in a local student symphony orchestra playing horn and wager tuba in pieces like Mahler 1, Alpine Symphonie, Brahms, Grieg, some Wagner or Bruckner 8. When I occasionally (2-3 time in 12 years) put a tuba mouthpiece to my lips it felt like putting my head into a bucket

. In 2024, year of the tuba

, I moved back to tuba. Within 2 months I played again tuba in our local community band. I felt, that my tone on the tuba was much clearer and clean, and full, warm, powerful as if I would have never moved away from tuba for 12 years. I also played some bass trombone which was also no issue in changing between the tuba and trombone. Now here is the experience: Then I tried my french horn with just a 18,5 mm mouthpiece and after 15-20 min, I was back on the french horn, sound and intonation wise. I was very very surprised about this. Now, although I play Tuba and bass trombone, I regularly also play french Horn and would not be afraid to play some pieces in front of an audience. Having spoken to a music friend who is also doctor, he explained, that my lip muscles and all the neuronal connections have now been trained for the rims of each instruments. My brain recognises the mouthpiece haptically and automatically "loads" the right muscle tensions. Especially the horn playing for 12 years has helped to develop the muscles in the center and at the tip of my lips, so at the end I have the stability for a smaller mouthpiece (horn) and the relaxation for a bigger mouthpiece (tuba), which interesting helps hight and low on both instruments. I do not know how common this flexibility is, but I can change from tuba to Horn in a Minute (have done this in a concert playing some horn solos in a piece between the tuba parts).
- I have no advice on a mouthpiece diameter which fits your french horn. I have felt every mm difference and could not have imagined at all before 2024 to play horn and tuba
- Maybe over a longer period playing both instruments in parallel, one could develop the same embouchure and muscle flexibility as I have experienced without sacrifying sound and intonation
- fun facts: I have lost my break between b and c in the stuff on the tuba, I play now a 31 mm Klier 7A on my Alex DoubleTuba 166 and a 37 mm mouthpiece on my older Alex 164 (an old model 37 Alexander mouthpiece nickel silver and silver coated what a sound despite the "smaller" bore of 8 mm and the quite shallow cup.
best
Chris