On most drillings, particularly drillings of some age, the bottom barrel is soldered to the upper barrels. thus produces the same thermal expansion as with double rifles.

In this case, the expansion will be show on target as a rise in impact, as you have experienced. When it has reached a stable temperature, shots will again group together.

It is a point inherent in soldered drillings and is only solved by free floating the barrels, securing them only at a couple of points to maintain consistent regulation. This allows more rapid cooling, so consistency is improved.

The krieghoff ultra (?) and blaser BD14 are examples.