To the best of my knowledge, it seems to be the milled Bulgarian AK's like the SLR-95. From what i've heard the SLR's have hammer forged barrels as well, supposedly forged by Styer.
What makes the SLR and other Bulgarian AK's "better" is the milled receiver, it is more rigid. The quality barrel is also an important factor.
I have the SLR and was extremely suprised that it shot as well as it did. I think I need to explain a bit more. So, here it goes:
Before I bought my SLR, I had seen A few AK's and was not very impressed. At the urging of one of my friends, an AK fan, I deceided I would buy one but I wanted a synthetic stock.
As fate would have it, the SLR-95's and Slr-96's were just starting to be imported. I tried other AK's but deceided to buy this on a whim.
I got some Federal American Eagle ammo for its first firing.
Now, of the other AK's i've seen, I can't honestly say they shot as well. I had a very good group the first time out!! No, it wasn't as good as my AR...BUT it was impressive for what it was. All in the black at 100 yards. I forget what rings, but it was reasonable!
My only regret.......I should have bought several others while I had the chance. As I live in California and they're banned now.
I don't know the availability of the SLR but there is another Bulgarian AK variant the SSR-99. It is also milled and may be more readily available, look around. Global Trades/Arsenal U.S.A. is the importer/manufacturer of Bulgarian AK's as well as some others using receivers from other countries.
I'm sure there are other guys lurking around here from AK-47.net since its shut down and they could probably tell you more than I can.
Now, regarding the question about the 7.62x39mm round versus the .223/5.56 The ballistics of the .223 make it more accurate. The 7.62x39 travels at a slower speed thus the ratio of distance traveled versus bullet drop results in a curve which declines more sharply than that of a bullet traveling faster, such as the .223/5.56
I hope this answers some of your questions!