[E. Pluribus Unum]

You can carry three times the amount of NATO 5.56 x 45 mm ammo versus 7.62 x 51 mm ammo. This is a factor that cannot be discounted, even if the lighter ammo is perceived to be less effective. The .223 is legal to use on deer, and it works adequately.

[El Gato]

The .223 is legal to use on deer

Depends on your location. Some states do not allow it for deer sized animals. How effective it is also depends on how big the deer are. Northern deer tend to be larger than southern ones, especially southwestern ones. And that's just the Whitetales, Mule deer are larger still.