The problem with the AR-15 chambered for 7.62x39mm is that the rifle was not designed for rounds with radically tapered cartridge cases. The AR's mag well is long enough to hold the first 5 or so rounds within its length, so that part of the mag has to be straight. The problem with 7.62 mags in an AR is that there has to be a transition from straight to curved at the bottom of the mag well, and that transition causes lots of feeding problems within the mag itself.
Look at the .223 AR 30rd mag. It's not very curved. Now look at an AK 30rd mag... curved like a banana. Put 10 rounds of AK ammo side to side (as if it were stacked in a mag). See the curve really start to develop after 5 rounds? That's why the mags HAVE to be curved for the 7.62 AK round... otherwise you'll get a big mess in the mag. All of the 30rd mags I've seen for 7.62 ARs were AK 30rd mag bottoms grafted onto straight sections.
In short, the reason these calibers aren't made anymore in the AR by the major players is because they aren't reliable past 5 rounds. If you want an AR, get it in .223 or a .223-based cartridge... or at least a non-tapered cartridge. If you want 7.62x39, get an AK.
Why is the 7.62x39 round so tapered? I think it's because the Russians wanted to use steel for cartridge cases (cheaper and lighter than brass, and just as suitable if you don't plan to reload it). Steel has a different elasticity than brass (less than brass), and doesn't spring as far, thus a straight case made of steel is more likely to stick in the chamber and cause extraction problems. We see this when people shoot steel-cased .223 ammo through their ARs... resulting in a lot of broken AR extractors because that steel case really grips the chamber and doesn't want to let go. The tapered case (and chamber) of the AK means that the round doesn't have to spring back much, and doesn't have to move back much during ejection, to break contact with the chamber.
