A well known fact in Germany. At firing one barrel of a soldered barrel assembly, be it a double rifle, drilling or combination gun, this barrel is heated and expands. This warming up measurably bends the barrels in the direction of the cold, unfired barrel. After a short time span the heat is dissipated to the still unfired barrel and the assembly straightens out again. In an open-sighted double rifle this movement is compensated as the front sight moves in the same way as the muzzles, so the effect of timing is unnoticable. As most German double rifles and dr drillings are scoped and a scope does not move with the muzzles, the time span between right and left shots has a marked effect on regulation. This is also the reason why most double rifles regulate either with open sights or with scope. See also Graeme Wright:"Shooting the British dr" page 188 of the 3rd edition. So a scoped dr has to be fired at the same sequence, right-left, and at the same timing, say 10 seconds, as the rifle was regulated. In practical hunting this timing necessity most often goes unnoticed, as the second barrel is used only at running game at short range or in case of an emergency when the animal does not drop properly at the first shot. The knowledge of this time to regulation effect is a reason behind the continental design of double rifles with ugly, usoldered barrels that are independent of each other.