Does anyone here have experience making gemshorns? I am trying to learn how and I have some questions about plug size, and how large the labium opening should be.

I started ... and failed miserably

However, one concept I did learn, practice with a suitable diameter piece of plastic pipe. This gives opportunity to experiment with labium sizes and angles without risking what might have been an expensive horn.

As I'm sure you're aware, a gemshorn is basically an early ocarina, so the final pitch of the instrument is relative to the internal volume of the horn. Best start a bit too large and bring it up to pitch by filling it a bit with beeswax or similar.

Good luck

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"I'm playing all the right notes—but not necessarily in the right order."