Well, best is debatable, but I think that most birds are taken with a shotgun, especially if shot on the wing.
The shotgun, with bird shot, is not considered a truly long range weapon. I see you are from England and I do not know your laws and what is available to you.
Generally a shotgun in the largest Guage you can shoot comfortably with a long barrel and a relatively tight choke will give the longest distance for birds. Action type, semi auto, pump, single shot, bolt is of course personal choice.