[Theohazard]

I highly doubt subsonic ammo would cycle the action in a Five seveN pistol. If you kept the bullet weight the same and just lowered the pressure, it definitely wouldn't cycle. And even if you increased the bullet weight drastically to make up for the lower velocity, the locking system on the pistol is specifically designed for fast, high-pressure rounds, so I doubt that would work either. Not to mention a significanlty heavier bullet wouldn't be stabilized by the rifling and would start to tumble pretty quickly, maybe even while still inside the suppressor.

So you'd most likely have to stick with regular supersonic rounds. And while a suppressor certainly takes the Five seveN's volume down quite a bit, it's still pretty loud due to the supersonic crack of the bullet.