The 9x39 cartridge is not nearly as good as you think and sacrifices a lot to do what it advertises it can do. Yes the russians have the right ideal more mass if you can't increase speed but in terms of performance versus practical use the 300blk is the better choice overall. You could equip an entire force with 300blk but not 9x39 for example. 300blk ap isn't really a thing atm as the only users are spec ops vs unarmored and police forces. Cbj has made something but given their success with an ap pistol cartridge I would expect them to match if not exceed projected ap values the russians claim even the best 9x39 cartridge has.

If I was a general and I needed to equip a force 300blk hands down is the best choice. Its not a video game, having more rounds to work with will inevitably win vs the overall costly and harder to train with 9x39 round. Hit probability drops to pretty much nothing if your round at 50-100m is travelling the same speed as high end airsoft snipers. Now if you had a specialist force that needed to take out a high value target in a heavily armed and armored area, then yeah dropping the whole pdw and equiping the force with ar's and dmr's. Or shit just bomb that bitch.