....What is "Ethical" in my view (and this obviously varies with the man) is a round that reflects a respect for life and your quarry. That is certainly not to say that 'if we feel that way we shouldn't kill the game at all. There is nothing unethical about acknowledging the natural food chain as wrought by nature's author. Is it unethical for a bear, cougar,weasel, ect.ect.. to kill and eat other life forms? Without taking life either animal or vegetable, our own is soon taken by nature. WE are predators by creation nothing unethical I can see in that!
....Do I wish to maim and un-necessarily inflict pain and suffering in taking the life that is to become my sustenance? NO! That is the measure of ethics for "ME". Each man must make this measure for himself. I killed a couple deer with a bow, then chose to give it up. I'd heard to many stories of lost animals around bow camps, and did not wish to have those experiences in my own memory banks. That's my choice, for me it is not more "sporting" to hunt with the bow, at least not for the game animal. My son John loves his bow hunting, and it would be un-ethical of me to enforce feelings born in me of sentimentality, and a kinship I feel with deer, on my son ,or on my fellow hunters. Each of us must judge such ancillary matters ourselves, while the right to hunt , the right to kill game, ought to be acknowledged in my view by way of studying natures law itself.
......Law ,true LAW,( not man's rules) is immutable, Ethics change with time & circumstance. In pioneer times, the children in need of food, winter bearing down, Mom and Dad hungry too, it wouldn't be unethical to take a deer, however much the beast must suffer, that isn't a necessary component of my hunts today.
).The accuracy of a smoothbore can also leave a lot to be desired. I won't trust a .410 on much of anything, that being said I occasionally enjoy them for squirrels and small birds. 
