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All recent film guns I've seen-- well, some aren't actually meant to be fired. One of the BARs in Saving Private Ryan, and perhaps some of the other weapons, were very realistic resin models so the actors could carry them (a) safely and (b) for the many hours and days of shooting to film one 10 minute scene.

But the ones that were meant to be fired began life as real no-kiddin' bullet launchers, modified to (a) accept blank cartridges made of .22 or 9mm (seldom larger) shell casings and (b) to reject efforts at chambering live ammunition. This includes partially blocking the barrel so that (semi-)autos can cycle.

It was not always so. In 1993 Brandon Lee was killed with a real pistol which had first been "fired" with a dummy cartridge that lodged a bullet in the barrel, and then fired with a blank which alone would have been harmless, but which pushed the bullet on out almost as well as a live cartridge.

http://en.wikipedia.org/wiki/Brandon_Lee

In Star Wars (1977), the stormtroopers' blasters were simulated by disguising a British SMG with plastic appliances. 9mm Blanks were then fired through them (the blaster sound added in post). In the cell-block scene, just after Han shoots the comm console, you can hear the 9mm casing hitting the floor.

I haven't seen any electrically-fired simulated gunpowder weapons. Doesn't mean there aren't any.