Burrard reported on a line of low velocity loads developed by the British Wildfowlers Assoc many years ago which they used with great success. The object was to be able to use a heavier load of the larger shot from a comparatively light weight gun for the shot load. For the ordinary 2" game gun the load was a 2 3/4 DE with 1 oz of shot, which could be fired comfortably from the average weight game gun. Smallest shot size recommended for this load was British #4 or the equivalent of our #5. Smaller size shot at these reduced velocities would not have adequate penetration on the larger size birds they were developed for. This game gun load was particularly good for high pheasant while most wildfowlers used a still heavier load from a heavier gun.
Loads of #7 or #8 @ 1100 to 1150 fps are perfectly adequate for the smaller birds a normal ranges. These loads were however designed to be used a maximum ranges for larger birds. Don't forget a 4 to 6 oz dove can fly untouched through a pattern which would put several hits on a 2 to 3 lb Mallard.
Splitting the middle in weight for both categories a 2 lb Mallard has about 7 times the body ares of a 5 oz dove.
effective range of a shotgun is always determined by the best balance for the game to be shot of adequate penetration & adequate pattern density to insure several hits on the bird. Yeah I am fully aware we've all made some kills we could only find one pellet hit on, but how many more one pellet hit birds fly off that show no sign of being hit at all.
