It is possible to take down a deer with a .223 and it has been done.One fack is that not every time you are going to pull the trigger everything is going to go right and your bullet is going to hit where you whould like it too.If you dont hit the deer in a major kill spot (Loungs,Heart,Neck,Head)and hit a spot other than a major kill spotyou are 85% more likely to loose a deer than you would be shooting a .270+ caliber.Smaller calibers such as a .223 dont open up a big hole and it will leave you with a small wond and the anamal will not bleed right.So in the end yes you can use a .223 but I dont recomend it.