Several times this year I have had other flutists asking me about my bass flute and whether I was able to play easily in the upper 3rd and into the 4th octave. My Kingma bass flute has a mid-size bore (sorry, don’t know the exact specs) and is able to play up to high C comfortably, high C# and D with effort. When I recorded Mark Barden’s Personae for bass flute and bass clarinet last year, I resorted to borrowing a Pearl bass flute that had a narrower bore, because I could never reliably hit a high E on my Kingma (which is otherwise an awesome instrument!). This passage is an example:

I am posting this to reassure you that if you are a seasoned bass flutist and are having real difficulty with these notes, don’t bang your head against a wall or berate yourself. Check your bore size. If you have one of those lovely large-bored instruments I really envy you – they sound marvelous! But I don’t envy you when you have to play in the 4th octave. Carla Reese sums things up nicely in her guide for buying large flutes:
“In general, a big bore instrument will have a stronger low register and a weaker high register than a small bore instrument. Bigger bores also tend to have a slightly slower response and more difference in tone between registers. Big bores are ideal for playing in flute choirs (especially for the bass) but can be heavier and need more air. Small bores are ideal for solo repertoire, where the demands can require more agility and a stronger high register.”

A colleague of mine who is a woodwind doubler has an extra small-bore Kotato bass flute, he says a high D pops out with hardly any effort.

I wish someone would invent a bass (or alto) flute that has an adjustable bore size!