I am no expert, but I have read in European, mainly French, souces that shooting technique can strongly affect apparent regulation.
Shots from the two barrels should be fired within a few seconds of each other. No addtional shots should be fired until the barrels have completely cooled down. This is the way European double rifles, i.e. "express rifles" are intended to be used on driven game and how they are regulated. Temperature effects can supposedly lead to what seem to be regulation problems if these procedures are not carefully followed.
Support can also be important as express rifles are obviously intended to be fired in an off-hand position.
