The barrel length measurement for a revolver is made from the muzzle to where the barrel terminates at the cylinder gap. It does not extend to the breach face. For your derringer (and for semi autos) the barrel length measurement goes from the muzzle all the way to the breech face. In other words, for a revolver it covers the actual barrel, for other types it covers the barrel as well as the chamber.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer