The bombshell news was just dropped in court today as part of crime scene technician Marissa Poppell's testimony in Alec's ongoing manslaughter trial down in New Mexico.

Poppell told the jury actor Jensen Ackles also had a live bullet on set ... the ammo was in his bandolier ... and he was on set the day cinematographer Halyna Hutchins was fatally shot.

Remember ... Alec's defense team, led by Alex Spiro, says it wasn't his job as an actor to make sure there was no live ammo in his gun. Alec's also said the gun went off without him squeezing the trigger.

Images of live ammo found on the "Rust" set were also shown in court Thursday ... and the bullets look strikingly similar to dummy rounds.

The only slight difference is live bullets have a silver center as seen from the bottom and the dummy rounds have a gold center.

So, even if Alec checked the revolver it doesn't seem likely he would be able to tell the difference between dummy rounds and live ammo.

Spiro asked Poppell if she had any reason to think Jensen knew there was a live round in his gun holster and she said no ... the implication being Alec would not have known about a live round in his gun either.

In addition to a live bullet in Alec's gun and one in Jensen's gun holster, live ammo was also found in a prop cart and a box of ammo.