Well yes there has to be some kind of rule system, but the parameters for hunting are the same with HFT especially, and most "discs" are larger than the "kill area" of the quarries brain. Targets are critter shaped most of the time and set at real life distances and angles, just like the field. Rats on ground at 15 yards, squirrel in a tree at 25 yards etc, which of them would you not take in the field.
I know it is not for everyone, but it defo makes a point if you have a go, and it definitely opened my eyes and made me a much better hunter with an air rifle.