So I’m just a little confused what is the actual point of the 9x39 cartridge. Supposedly it’s meant to be subsonic so it can be suppressed. But why is the case so long? 9x19 is typically supersonic unless loaded subsonic or the gun uses a ported barrel to reduce velocity. So if 19mm of case length is enough to get a 9mm bullet over the sound barrier, why would one want a 9mm cartridge that’s 39mm long if it’s going to be fired at subsonic velocities? Is it supposed to be the same idea as .300 blackout where you can have both subsonic and supersonic loadings?

Its not the same, in any way, as a 9x19.  
wolfganggross

It's a rifle round, not a pistol round. The bullet used is long enough that it wouldn't fit in a shorter case. It's also standard length as to run in existing 7.62x39 mags. It is a full power rifle cartridge, designed for suppressed use and armor piercing capabilities. The penetrative ability of armor piercing 9x39 is (at least in Russian documents) very impressive. Basically the Russian 300 blk. He have 556 case standard, so 300blk is a necked up 556. Russians have vast stores of 7.62x39, so thats what they chose to neck up. Case length stays unchanged

147gr vs 278gr. Small difference.

Yup. It's at least 2X more "powerful" by every measure, not including AP capability.

And in my limited understanding, it is superior to our 300BO because of some sub-freezing temperature environment performance. Basically the Russians figured out how much ass (power, bullet weight etc) was needed to stay subsonic at +100 to -30, yet still function 100% reliably. Conversely, our 300 cannot accomplish this even with the heaviest 220gr pills. Or so I'm told.  

Maybe the 350 Legend has some potential but I'd rather just get some guns and ammo in the 9X39 personally.