My draw to 9x39 is mostly on the combloc / AK side, specifically the AS-VAL, but since those aren't available yet I dabbled in the AR world in the meantime
From my research it seems like that while 300BO supers may get the job done, the subs don't reliably work for hunting. The 9x39 was designed to be effective subsonic all the time and I as I understand it it might be a better round than 300BO subs. The Russians encountered the same kinds of problems with 7.62x39 subsonic bullet weight as 300BO and seemed to feel that the bullet needed to be significantly heavier than 220gr to be reliably effective at subsonic velocites. They settled on 270gr + and have apparently been very happy with it
In my case I wanted specifically to shoot subs for hunting so I took the plunge on 9x39
My 10.5" barrel from Piedmont cartridge has been holding about 1.5" at 50 yards with a 4 MOA red dot and I'm happy with that so far. I'm hoping that the cartridge takes off and we get some better bullet types, but at least Tula FMJ is about 60 cents a round now which isn't terrible
9x39 with Ghost M on the bottom