I have been told that the ideal length for a 5.56 Nato barrel is 20" and I'm wondering what the best length ballistically speaking is for the 7.62x39 round is?

Thanks,
Jim

meh, 20 or 22" IMO wouldn't be much better then 16", you may get a few hundred extra FPS if your lucky but accuracy i don't believe would improve, but hey, i could be and probably am wrong. So feel free to correct me, but i think it comes down to skill and sights. I had a 20" AR and it shot great, but not any better then a 16" one...

The longer barrel generally just improves the sight picture, which increases accuracy with iron sights.
FPS will drop a bit on the 16", and muzzle flash will decrease on the longer barrels.
It's all a matter of preference. I like 'em short n light (women and firearms).