I'm not an armorer, but I'm wondering if the reason you don't see many P99s in films is because theyre going to get shot alot, and P99s wear out quickly compared to other pistols. I was talking to a guy who runs a rental range, and who's guns see alot of rounds, and he said the P99s were always the first to need replacing. CZ75s last the longest, if anyone was wondering.
I know the question was meant in a film context, but here's the BFA for my C7:
The open section at the back locks around the groove in the flash hider (originally meant for rifle grenades) and then you screw in the plug to block up the barrel. You can even see how filthy it is in there from all the carbon build-up - they come pained safety yellow. Unit SOP is to cam it up with green gun tape though.