Thanks Gentlemen, and in particular Niklas - you have confirmed my thoughts. I have also spoken to one of the manufacturers of Comobination guns and they usually expect 2nd shot to go 10 to 12 cm high from a warm barrel if barrel is soldered to the other barrels. As the barrel heats up it lengthens slightly causing to warp upwards.

Make sense that lighter load with less powder / energy will cause barrel to heat up less.