Shell ejection isn't something you want to be bothered with. Overall, it'll be cheaper and easier to add some in post. You can even cheat it effectively by simply adding the sound of brass hitting objects/ the floor, because film doesn't pick up much shell action anyway (brass flies *really* fast).
If you want to do slow mo stuff you need to hire an Armourer, no real way to get around that unless you have a real hotshit CGI guy who will work for free.
High quality plastic guns like Tokyo Marui products should be considered. They have full trademarks and the camera can't tell the difference. Many metal guns look more like an airsoft gun than a TM does, by a long shot. If your talent wants metal guns because they think that will help their craft, tell them to harden up and act better.
