A superposed barrel gun should always fire the bottom barrel first, IMO.
SRH
Stan, I know it is what everyone says, but would you mind explaining exactly why? I shoot a couple o/u guns, primarily for hunting. I always shoot the top barrel first. My flawed logic is, if I only fire once, I can more easily reload. I find reloading quickly a benefit when waterfowl hunting in case I need to water swat a cripple and grouse hunting provides regular opportunities for delayed flush shots. I've tried bottom barrel first but don't appreciate any significant difference while shooting.
Steve