if you want a 7.62/39 then buy an AK. You can pick up an AK for the cost of a good barrel / upper for an AR-15. There are a lot of great choices out there for good barrels and uppers. I have a bull barrel from Double Star that shoots wonderfully. Fairly reasonably priced too. YOu can always go big, and buy a douglas, too.
As far as 5.56 Nato, and .223. I have shot both from my rifle with no problem.