AS hokey as it was, the Van Damm movie "Double Impact" had a really neat scene where Van Damm, fighting the usual hordes of bad guys, grabs a dead BGs M9. He racks the slide (presumably because he is not sure if it is loaded or not) which ejects a round!
IIRC, in "Jason X", the girl in the begining racks a shotgun twice, ejecting a shell the second time.
I think directors and editiors add lots of "mechanical" sounds to firearms to A) draw the viewers attention to them, B) mask the fact that most on-set weapons are rubber, somehow they think that by making the gun rattle, it will "feel" more real to the viewer, and C) appease the moron viewer who thinks that ALL guns have a safety that has to be disengaged, a hammer that needs to be cocked, and a chamber that has to be loaded BEFORE any shooting can begin.
Hollywood never really gets ANYTHING right. Phones don't immediately go to a dial tone if someone hangs up on you, elevators don't keep closing if a guy snakes his arm through the doors, hot rich chicks don't stay single till they are 30, bullets in the shoulder can KILL/CRIPPLE you, police don't usually fall asleep when on patrol or guarding someone with active death threats against them, gas tanks do not routinely explode when shot, and air vents are NOT large enough to crawl through. But all of these things are an accepted part of the Hollywood universe and the poorly researched scripts that dwell therein.