John

To the best of my knowledge you are correct, I have never seen, read or heard of multiple barrelled guns used here (that is not to say they did not ever?)

The number of shot/pellets in a single barrel would exceed multiples (unless each barrel held many shot)
and would be unnecessarily expensive and complicated, even Lancaster's multiple "Russian" 12 bore

The very specialised guys doing this type of shooting would stick with what they knew worked, although the Continentals could be otherwise

I have read the Somme Estuary was a prime site in years gone by

Tony