Well, to be completely honest here, the P90 DOESN'T use 9mm rounds. It uses hard-hitting rifle-like 5.7×28mm rounds. I agree the P90's damage should be nerfed by 4, to make the game "more realistic", but hey, it's a game. The Luger is literally a 9mm sidearm but it deals more damage than the AK.
As per se, since the game must be COMPLETELY BALANCED, I think the weapon DMG is fine as it is right now. While the AK IRL does use 7.62x39mm rounds, the 5.56mm rounds of the M16 (and the HK-416) actually has more velocity, and, theoretically, more hitting power, so the AK should just have 35 DMG. That'd be completely fine by me. Now, what I DON'T GET is why the HK has "harder-hitting" penetrating rounds. Also, one thing you got wrong is that the AK of THIS GAME is really indeed an AK-47 and not an AK-74, due to the AK in this game having a "milled receiver", according to trivia.
The SCAR-H, in comparison, actually uses a different round than the AK. It uses a 7.62x51mm round which is MUCH, MUCH harder hitting than both the AK or the M16A1. As such, I believe that a +5 DMG increase is necessary for the SCAR, but 55 DMG per shot for the SCAR-H would make it totally overpowered and would ruin the game, just like the FAMAS ruined the game back in the old 2012 R2D.
But, hey, it's a game, and everything has to be balanced.