After having F.W.Heym make a 60cm long set of O/U barrels for me, I had the honor to watch Helmut Kerner and Rudi Henneberger test and adjust the regulation. They used a reamer with a long handle that one of them turned from the breech end, while the other one "crowded" the reamer to one side with a brace that looked like a yoke. The "magic" was in knowing where to apply the pressure and how much to apply. It worked; I killed a pickup load of game with those barrels.
Mike