Don't forget its gun proof ammo, muzzle pressure is pretty much high and any minor changes in proof ammo loading could be fatal. For instance lead shot hardness of every pellet and its size.
In 70-th of last cent. Germans began to use parabolic full chokes in Merkel o/u's gun as it was used in Russian sporting guns such as MC-8, these chokes gave extremely tight patterns when small shot numbers were used. The result was ring bulges on most Merkels, when large shot numbers were used, because MC-8 was heavy gun with massive barrels, when most of Merkels were light weight guns.

Geno.