Some prop guns often fire a blank and substitute a small charge of flamable oil for the projectile, causing a muzzle flash easy to capture on film. It is even worse in person, from what I’ve read, and is why most actors have a horrendous flinch when shooting, they’re used to a big flame blossom every trigger pull.
Another type of prop gun runs on propane, making a similar flash and a nice “THUD”, but a lot of directors are going to realistic Airsoft replicas and adding flashes/brass/etc in post entirely.

None of these options lead to realistic movie/TV guns.