40 yards is the normally accepted range of a shotgun for clean reliable kills. Always has been and with modern steel still is. It is the equivalent of 200 with a stalking rifle.
There are some who can consistently kill live birds a bit further - 50 perhaps 60 at a push, but we are now talking heavier loads and guns and good shots. What is not discussed are the number of pricked birds that are never recovered.
There are also those who break clays at even greater ranges - I have seen videos claiming over 100, but its not consistent and a clay is lot easier to kill than a bird.