My main shooting buddies will absolutely not shut up about how "impractical" of a deer rifle my 20 inch AR-10 (308) with 5-30x scope is.
If I had a desire to kill a deer with it, that deer would DIE. But, uh, not exactly the point.
(Edit to elaborate/clarify: I don't have any particular desire to hunt, so I am annoyed that my friends insist on discussing the suitability of my long distance precision target rifle for hunting. That said, if I indeed did desire to hunt with it, it would be more than adequate for the task)
That's not what the OP said, and 223 and 308 are both capable of painless kills with proper shot placement. Head shots are highly discouraged in hunting, regardless of the caliber.
In any case, this isn't the place to debate veganism or "ethical" hunting. If you would like to do so you can make another post about it.
I've never been hunting, why are headshots discouraged? Is it because they're hard to land and easy to botch?
Yeah, heart and lungs are a much much bigger target, every bit as deadly, and not completely surrounded by extremely sturdy bone.