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Mouthpiece drill (smallest point of MP). - A
Smallest lead pipe diameter. (where the receiver and LP join?) - B
Instrument bore (valve bore/slide bore?). - C

A 8.84mm
B 12.7mm
C 18.5mm

What's your combo and does it really matter?

Mine's sort of roughly conical, so it's kind of complicated. How did you measure B?

With an internal caliper gauge pushed in from the MP end, so I did assume that was the smallest dia.