Due to the feed issues caused by the distinct taper of the 7.62x39 case I would suggest you look into doing a 300 Blackout instead. While the 300 BLK loses between 100 and 200 fps when compared to the 7.62 x 39 it's a real sweetheart of a round to reload and you can use standard AR magazines without any issues with jamming.

BTW, I have a 16 inch 300 BLK I built myself with a Wilson Combat barrel featuring a pistol length gas system. To allow me to tune the gas system for whatever ammo I am using I installed a Syrac Ordinance adjustable gas block. Doing this allows me to dial back the gas supply to keep the rifle from being over gassed with supersonic ammo and a secondary benefit of this capability is that it's so clean shooting you have to actually see it to believe it. With 100 rounds downrange I can pull the Nickel Boron plated Bolt Carrier and it looks like no more than 5 rounds have been fired. I will also note that the most accurate powder for my rifle has been H110 and it will consistently put 5 rounds in 3/4 inch at 100 yards with either a 125 gr Nosler ballistic tip or Hornady HP.