In ancient times, when I was in basic training in the German army, we learned the safety distance for a G3 (7.52x51) was 3000m, the effective combat range of the same gun when mounted was 900m.
I don't know at what point the projectile goes transonic (and starts to tumble), but I guess it's probably at 2000+ m.
I heard the AK47 has a somewhat shorter barrel, so the bullets should be a bit slower, but they still should reach up a few thousand feet.