The smoking gun that claimed the life of Halyna Hutchins might've been more than just an on-set prop -- it was also being fired recreationally, even when cameras weren't rolling.
Multiple sources directly connected to the 'Rust' production tell TMZ ... the same gun Alec Baldwin accidentally fired -- hitting the DP and director -- was being used by crews members off set as well, for what we're told amounted to target practice.
We're told this off-the-clock shooting -- which was allegedly happening away from the movie lot -- was being done with real bullets ... which is how some who worked on the film believe a live round found its way in one of the chambers that day.
Obviously, authorities would want to speak to anyone who had been using the weapon for target practice, and -- more importantly -- to whoever was putting the gun back among others used for filming.
We've reached out to law enforcement to see if that's happened yet.
There's also this ... one source who was on set and familiar with the goings-on of the crew tells us that when cops showed up, they found live ammo and blanks were being stored in the same area -- another possible explanation for how an actual bullet slipped got in the gun.