7.62x39 basically hits like a 30/30 with the accuracy of a bolt. Higher BCs means flatter trajectories and better performance over those medium distances. The rounds get there faster and "float" much better than the 30/30. With your run-of-the-mill 30/30 factory ammo, you just can't reliably do this:
https://youtu.be/caCiANMCixo with a 30/30: believe me I've tried. An accurized 30/30 running handloads might come close, but we're comparing off-the-shelf setups right?
There's a reason it's a military calibre and that's because it's a superb all-rounder. Unfortunately cheap bulk 7.62x39 is not a thing in AUS anymore, so like all ammo these days, it's frustratingly expensive, almost the same as 308. So that somewhat negates its utility (in AUS). But for OP (who has specific needs), I think it's a suberb choice.
Another bonus is there's commercially available cast lead projectiles around for .312" grooves - excellent (and cheap) practice round over TB.