As Kudae mentions, in the regulation of a double rifle, the first shot warms and expands the first barrel, this has a stretching affect on the second barrel. The quantification of this depends on the amount of time taken to make the second shot.

Paco, the German gunmaker at Heym who one of the principal gunmakers responsible for regulating their double rifles, says he regulates for a period of seven seconds. A longger period will cause the barrels to spread further.

It is interesting to read Kudae's comments re open sights, vs scopes, with scopes there is greater influence of the stretching of the barrels during warming increasing barrel spread, compared to open sights.