[yellowapple]

> And I'm told that french horn players use the hand in the bell as a tuning slide.

You can do this, since how far your hand is in the bell can determine pitch up to even a whole semitone or more (I forgot which way, though, but I think it goes flat as you shove your hand in further; it's been a few years since I've played French horn on any semblance of a routine basis, and I don't have one on hand (those things are damn expensive...)). However, this will also affect tone; the further you shove your hand in, the more muffled it'll sound.

French horn players usually opt for double-horns (F/B-flat) in order to avoid needing to use their bell-hands for tuning; since both "sides" (the F side and the B-flat side) of each rotor can be tuned independently of one another, it means that a skilled French horn player who's learned how to transpose F-keyed notes to B-flat-keyed fingerings (which is actually pretty easy if you already know how to play a bass-clef'd valved brass instrument like a baritone or euphonium or tuba, since F-key and bass-clef C-key have the same note positions on the staff, so you just need to be aware of various differences in accidentals and you can transpose very easily) can pick either an F-side fingering or a B-flat-side fingering for a given note depending on which one is closer to the desired pitch.