If your goal is an ethical, clean kill, you would be well served to use as large diameter a bullet as you can handle that will perform well at the ranges you intend to shoot. The factor that makes a bullet effective in "dropping" an animal, is the transfer of kinetic energy from the bullet, to the tissues and organs of an animal. Allot of people tend to think that if a bullet leave a massive wound channel, shattered bones, and a gaping exit wound, that it will effectively drop the critter. I've seen some wounds in whitetails from 7mm magnums that look for all the world like "lights out", but the deer made it several hundred yards before lying down. Small bullets rely on speed and fragmentation to transfer energy, and do so in a very concentrated area (hence the shredded wound channel). This is most definitely lethal, but not reliably quick. I consider .30 caliber to be a good minimum for ethical deer hunting, and have only used an AR once when I was younger, dumber, and more prone to listening to people who make hitting the spine and jugular vein of a living, moving animal sound easy, and reliable. I also advocate using round nose bullets for any hunting under 400 yards, since they tend to hit like a truck, and leave a very clean, humane kill. I now use a .375 H&H (I know some of you will get a hoot out of that), which does less tissue damage than any other rifle I, or anyone I hunt with have ever used. My brother shot a deer with it two years ago, and you could visibly observe the body cavity expand when the bullet struck, and the animal fell where it stood, and was expired by the time we reached it. This round leaves any un-struck organs intact, but tenderized, and mushy feeling (like everything inside the cavity got mashed with a bat. I don't suggest that this is the only, or best type of weapon to use, but it fits my purpose very well. I do not go out looking for a challenge in killing any game, I make sure I have enough gun, and I pass on questionable shots. I do think that unless you live in a part of the country where the deer don't get much above a hundred pounds on the large side, the small caliber rifle game has more to do with treating the hunt as some sort of game, where making something more difficult than it needs to be gets you extra points.