Surprised no one has mentioned the continental guns seem to be heavy in 16ga which is superior to the 20 and gives little away to the 12ga except in waterfowl. I haven't hunted wood pigeon but the 20 does well enough on Rock Dove (pigeons) and pheasant that I don't see any reason it wouldn't work on the wood pigeon. Haven't hunted European hares either, but at 8lbs they aren't as big as antelope jackrabbits and my 20 has never failed on them. Haven't hunted European woodcock either but ruffed grouse and US woodcock are perfect matches with the 20 and open chokes. So in my opinion the reason for the selection of gauges in the various countries is probably based on factors still undisclosed.
