Well I happily stand corrected on the Piston upper manufacturers (dedicated uppers that I cannot find), but disagree with the sentiment that there is no rational for 7.62x39 in the AR platform. I don't get to make manufacturing choices and my opinions are just that.. mine.
I'm sorry, I guess I did not make myself clear. I have no problem understanding the need for an AR platform in 7.62x39. I own 2. There is a need for this. My comment was religated to the gas-piston operated AR in 7.62x39. The reason I do not see the need for that particular type of AR in that particular caliber, is that you have just turned an AR15 into an AK that has a different set of recievers. I will grant you that the AR, even in the gas piston system, will be more accurate than a similarly outfitted AK just simply by the nature of the locking lug bolt system during firing, but the nominal advantage gained in accuracy may be lost in reliability unless you have a dedicated AK magazine lower. Which again, is simply making an AR into an AK...
Though there are several of the newer generation assault rifles that have been designed to accommodate a operator level transition to 7.62x39. Specifically to my knowledge those would be the FN SCAR and Bushmaster/MagPul ACR platforms (I'm also thinking the XM8 was designed with this option in mind as well but could be recalling incorrectly). Knights Armament Co. had one on the boards for a while, I don't know what happened with that effort, but it would have utilized standard AK mags, hence a different magazine well, which would equal a different lower receiver.
...at which point we no longer have an AR15, but an AR look-alike.
Again, I have no problem with the caliber. Short range/CQB, it is a superior round to the 5.56x45 within the limitations of the Geneva Convention. But, it just seems like a great deal of work to recreate what we already have in the proven and reliable AK-47. But hey, if somebody wants to spend the time and money, I know they'd have a great system in the end. I just think it is another invention of the wheel is all.
MGI also offers 7.62x39 in their modular AR line which requires the mag well to be swapped out rather than the entire lower. Again another company that was rumored to have a Piston iteration of their weapon system.
Has anyone seen one in person? I have not. I know others who have had them on order for a very long time and are still awaiting delivery. It's a grand concept, but does it work? AND, if it does you'd still have a standard AR operating system... unless you pourchased the gas pistl conversion system.
- Gas system $600-$800.00
- MGI lower $400.00
- Basic AR15 carbine $850.00
- Dozen AK mags $150.00
- Now you've got a really cool AR15 converted into an AK47 for $2,100.00!!
We also can't discount the piston conversion options for the AR platform from the likes of Ares and a handful of other vendors. These in conjunction with a 7.62x39 upper and AK mag accepting lower would make for a more accurate weapon in that caliber... Perhaps something for you to mention to the R&D guys

I know I'd consider one.
The RRA 9mm for my part was bought at an excellent price and has performed very well for the uses I originally described. The manual of arms is exactly the same, the ammo cost is lower and with honesty, this would be a weapon I would find acceptable for close quarters use (sub 25m).
Agreed. No argument there! Those pistol cal carbines are fun. I have a 10mm, and just sold one of two uppers in 41 AE ever made!