Inception - Cobb's Pistol

Just saw Inception last night (bloody awesome movie btw) and was intrigued to see at the start of the film, when they are inside Saito's dream, Cobb puts a suppressor on his pistol (which I since learned was a Beretta Px4 from this site) and proceeds to manually work the slide as he takes down Saito's guards.

Now, I'm aware some semi-automatic pistols are designed (or can be modified) to have a slide lock so the weapon doesn't cycle, specifically so firing with a suppressor is quieter - not sure if that's the case with the Px4 or not - but in the movie it looks like he's actually working the slide to fire a round, which is weird.

I don't know if its just a mistake on the director's or sound editor's part, but as Leo lifts his weapon and pulls the slide back, it flies forward and then you hear the sound of the weapon firing - rather than lifting the weapon, firing it, then working the slide.

Anyone else notice this?