Alec Baldwin was filming a scene for a movie that required someone to fire a gun loaded with blanks … but when the gun discharged either shrapnel or a bullet hit 2 people on set … one of whom had to be airlifted to a hospital.

Our sources tell us the incident happened Thursday on location in Santa Fe, New Mexico, where Alec is starring in a new western called “Rust” at a movie ranch called Bonanza Creek Ranch.

We’re told Alec was filming the scene when someone pulled the trigger. It’s unclear if the person who loaded the gun mistakenly placed bullets inside, or if something was lodged in the barrel that hit the director as well as the director of photography.

Our sources say the director was hit in the clavicle, but the DP wasn’t so lucky … she was airlifted and rushed to a hospital. As for Alec, we’re told he was not injured.

Earlier Thursday, he posted a pic of himself in character on the set. After the accident, we’re told production shut down for the day.