The 5.56 possibly creates a higher chamber pressure and is why it's not suggested to use a 5.56 in a rifle labeled/chambered in .223 and why it is perfectly fine to use .223 in a 5.56 chambered barrel. They are technically interchangeable.

Depends on your deer too. But I agree a .22 and a 30-06 would probably prove much more useful.

There can be a difference in .223 and 5.56mm factory ammunition at least when fired in some rifles. This quote is from the Bushmaster Q&A section:

"The NATO Spec 5.56mm chambers have a longer "leade" or throat than the SAAMI Spec .223 caliber chambers which have about half of the leade or throat of the 5.56mm chambering. While it is safe to fire both 5.56mm and .223 caliber ammunition in our rifles, the 5.56mm ammunition should not be fired in rifles chambered in .223 caliber as they will develop very high pressures."

http://www.bushmaster.com/faqs/afmmain.aspx?topicid=&faqid=&question=

Like you I have fired handloads using 5.56mm military cases in AR type rifles with no problem.

I like several things about 5.56 for a survival rifle. You can take game up to and including deer, you can carry more ammunition for a given amount of weight than with heavier cartridges, it's probably the most plentifully available rifle ammo in North America, and it's reasonably effective for defensive purposes.

There's no perfect firearm for every survival situation; all are compromises. I'd certainly prefer a .22LR for taking small game and a .30-06 for taking deer or anything larger.

Like the others above have said 5.56mm ok to use .223. if it's marked .223 5.56mm is a no-no. if I remember it has to do with chamber pressures. Check you barrel and see what's marked on it.