Edit: I got a better look at the surveillance video. Looks like what is actually happening is that the guy's gun is under-gassed. The rounds aren't producing enough energy to fully cycle the weapon, resulting in jams or requiring him to manually chamber the next round. There goes the professional international assassin theory, I'd think a professional would test their set up to make sure everything was adjusted to cycle properly.

The way these are set up - When you fire a normal semi-auto pistol the energy from the shot pushes the slide back so the gun can chamber a new round. You can then fire again immediately.

With these things the slide is locked closed when you fire. That means the slide doesn't move, which means it doesn't make noise. All the gas goes out the barrel through the suppressor. The brass stays in the chamber.

All of this combines, with the right silencer and ammo, to make a gun that is ridiculouslly bad action movie quiet. If you're standing right next to it you'll hear something but it's not going to sound like a gunshot.

Idk what the hell is going on here but it sure does look like some cool John Wick level shit.