I've read about the protracted attempts to create a suppressed, subsonic 5.45×39 AK-74. It would be used with special subsonic loads, just like the suppressed 7.62×39 AK with the BS reduced subsonic ammunition.

The article said that the project was unsuccessful. That the high-velocity small-caliber 5.45 has much greater bore friction; it's much harder to push the light, small bullet with a hard jacket through the barrel with tighter tolerances, to get rifling engagement.

Normal loads overcome it easily and accelerate the projectile to 850 m/s. But with the reduced loads, they struggled to even clear the barrel. Or rather, if the barrel was already used (not shot out, just had slightly widened tolerances due to normal-to-heavy use), the velocity shot way beyond subsonic, so useless. But if the barrel was new, it barely got out, sometimes even getting stuck in the barrel as a squib. The lightness of the 5.45 and high friction brought it to the very (bottom) edge of performance.

Obviously this doesn't fly, the gun/cartridge system becomes completely unpredictable and ballistics are all over the place, plus it's a crapshoot if the suppressor even works.

I don't know if it would help if they made it a malleable lead bullet like the .22LR, but anyway, the project was abandoned.