I don't know where people get crazy ideas like this. They make deer out to be some sort mythical creature that can't be killed with anything but a 30-06 or bigger shooting heavy bullets. Next thing they will say is they need silver bullets

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You could kill a deer with field tipped arrow if you really wanted to, but a broad head is more effective. If you want to argue that bows kill differently than guns (which they don't in my opinion) then I know plenty of people that have taken deer with a .410 slug gun. I might even try one this year, I have a winchester 9410 just itching to kill something.
To think that any high powered rifle is incapable of killing a deer is just ignorant. With the proper bullet, proper shot placement and keeping range in mind a .223 is plenty capable of killing a deer. They are just not that tough of an animal. I wouldn't try shooting one in the shoulder at 200 yards with varmint ammo though. I think that is where people mess up, they don't take the limitations of the weapon into mind.
Here in Michigan we have pretty lax requirements for hunting weapons, especially up north. I bet plenty of deer have been killed with AR's using 223 ammo. I know of a few people that hunt with AR's and SKS's.
I'll be honest and say it wouldn't be my first choice, but I wouldn't be afraid to use if that's all I had.
Paul