While I am not a balistician I think that the major reason the light shot losds are loaded to a high veloity has not been mentioned. This is to give a high enough pressure to ensure adequate burning of the powder. "Dirty" burning is not really a great problem "Unless" it is an indication of burning at a lower pressure than the powder was designed for. Stuck wads left in a bore are a great problem.
It is my understanding that when the 2" 12 ga was developed in England the original intent had been to load a 3/4oz shot load. No suitable powder of that age was found for the 3/4oz charge so a 7/8oz load was settled upon.
"IF" you are laoding a powder which is ordinarily clean burning & upon lowering the ballistics it starts burning "Dirty" be extremely cautious with those loads & look down the bore after every shot.
