The full-auto MAc-10 fires from an open bolt, which makes it vulnerable to slam firing - but that's an issue to do with the way the firing mechanism and magazine are set up, so it might go off when reloading, or burst-fire when trying to fire single-shot. It's *not* the same as going off when dropped - that's not what "slam-firing" means.
In the film, this is precisely what happens, iirc. Jamie lee fires the Mac, panics, and drops weapon, which is still firing as it falls
, even before it hits the first step. They even mention the first part of this on Mythbusters, but neglect to note that the gun in the film is already firing before it ever hits the stairs. Which is why I wondered why they were at all concerned with whether a drop impact would cause a gun to fire, as that's not at all what the film depicted.