7.62x39 in AR platform has been around for a while. however my experiences there were not positive. to me if i wanted a .30 cal AR i would just go to a 300 blk despite the 7.62x39 being more powerful. the only different parts are the BCG, bolt and magazines. From what I can tell it uses similar buffer and spring. i think the gas port is closer something like carbine length but not a major difference like a 300 blk that runs best on a pistol length.
issues many encounter:
-poor feeding through magazines, due to tapered cartridge going up a straight mag well
-lighter hammer springs lead to lightstrikes on the berdan primed ammo, so most end up running mil spec or heavier hammer springs
-due to larger chambering, the walls of the bolt face are thinner. i frequently shredded off bolt lugs, must faster than on a comparable 5.56. i'm guessing this is why CMMG went to a larger bolt/BCG on their mutant 7.62x39 rifle
-increased degree of fouling in chamber and BCG particularly with steel cased ammo (this is my own observation, may not be universal)
honestly most who get into this setup end up wishing they went with a 5.56. in my case i pulled out the 7.62 barrel and placed what shoudl have been there from the start, a 16 inch BCM lightweight barrel. for longer range a heavy 223 just walks all over a 7.62x39. yes this is a sensitive topic but most who love the 7.62x39 never shoot past 200 yards. i still dig the cartridge but i find 223 far more versatile.
to have a truly reliable and smooth operating rifle in 7.62x39, it's best built around a curved magazine like an AK. just my opinion. it's worth exactly what you paid for it.
A friend showed up with a new to him AR chambered in x39 the other day and none of us knew that such an animal existed (very familiar w 233/5.56 and 300 blackout). He bought it used and was not familiar either. Seems to be custom mags that fit standard lower. 
