I think the point is not whether they are SxS or O/U, but that they have the barrel configuration (length, chamber, choke) to handle the larger loads of powder and shot, to achieve the killing distances we are talking about.

My guns are SxS, English, 2 1/2" chambers. I wouldn't want to put anything more than 32g of No 5 shot through them. That's more than enough for the type of driven shooting I am fortunate enough to do.

So if I go on my mythical high bird day, I'd have to get a MK38 with 30" + barrels, 2 3/4" chambers and a lot of practice! Clearly, these days, most modern guns that can take those loads tend to be O/U; laws of supply and demand.

Tim