knayrb said:
Deer move so fast after being hit that I doubt you would get a second or third shot to hit it even with a semi-auto .223. The distance between hit and drop is going to be a long walk with a .223 IMO.
Which brings up the biggest issue with hunting with "light" calibers for the game: Not only will a deer move fast, and a great distance, small holes generally mean little external bleeding. That means not much blood trail to follow, which means that you may well lose the trail before you get to the end of the long walk.
Put that .223 right through the lungs or, even better, the heart, and it won't matter. Lung shots will give you lots of frothy pink blood to follow, and heart shots will stop the animal almost in its tracks. Braincase or spinal cord shots will obviously drop the animal in its tracks with any legal caliber. But put the .223 through large muscles or through the gut and you're going to get a long, slow death with little bleeding (on the plus side, if you DO find the animal, you won't ruin nearly as much meat with a .223 haunch shot as with, say, a .30-06). Hit a large bone with an 80 grain bullet and it may just lodge there, where a 150-grain bullet at good velocity (2000+ fps) will shatter a shoulder, hip, etc. That may not kill the animal quickly, but it will STOP it, allowing you to finish it humanely with a head shot.
And if you're going after elk, with their much more massive bodies, you need an even better stopper.
I would not recommend hunting mule deer with a .223. It's legal, it'll kill them, but a heavier cartridge will do a better job.