I like the 7.62 X 39. My Yugoslavian AK-47 is the only rifle I have in that caliber. I have four rifles in 5.56 mm. The ammo is easy to come by. I much prefer the 7.62x39 as it makes a bigger hole and doesn't require ear muffs to shoot. I am considering getting a RRA 6.8 SPC upper for one of my AR-15s, but I am not sure that the round has caught on. I heard that SOCOM was using it. I am sorry to hear that the 7.62x39 ammo is getting scarce. I had thought about getting an upper in that caliber. BTW, AP is Armor Piercing.