Welcome to the forum, Kynoch.

It seems strange to me that you encountered 3 modern doubles that wouldn't hold better than 8-inches at 100-yards! I believe you were just unlucky: most will do much better than that. Most old Brit SxSs, even those with a century of wear and abuse, will do better than that! I wouldn't give up just yet!

Most of us here would prefer the SxS configuration, it has a number of advantages in the hunting field, but the O/U (if you must!) is probably at its best with the 9.3x74R chambering.

Mind you, if you were to abandon the search for a good double, you could certainly do a lot worse than the 9.5 Mannlicher Schoenauer!