Most Brass instruments have either a Conical or Cylindrical Bore, so what's the difference?
The bore is the diameter of the tubing used and normally measured in the centre of the instrument around the valve block.
This is where the bore diameter is consistent throughout the instrument until you get to the bell section at the end where it flairs out. This gives a well projected, direct sound and is typically found in instruments such as Trumpets and Trombones.
This is where the bore is cone shaped and the diameter gradually increases throughout the entire length of the tubing, providing a much fuller/warmer sound than a cylindrical bore. This bore is found on French Horns and most Brass Band instruments such as Cornets, Euphoniums and Tubas.