We talked to Steve Wolf -- who's been working in SFX and props in films for several decades -- and he laid out exactly what he thinks went wrong on the New Mexico set where cinematographer Halyna Hutchins was accidentally shot and killed.

Check out our convo with him on "TMZ Live" ... Steve says the lead armorer, Hannah Reed, was not experienced enough to handle the job, in his opinion.

He explains with that type of role on a set -- one of the most important ones, especially if weapons are at play -- it requires a certain authority and expertise he believes Reed might not have had.

He also says low-budget flicks -- which "Rust" appears to be -- often do a lot of corner-cutting ... like not waiting on actual prop guns, or opting for live ammo or not taking the extra steps to thoroughly check weapons before they're used.

Finally, he addresses the burning question on everyone's minds -- who's actually responsible, and should Baldwin himself shoulder any blame? In Wolf's eyes, he does ... to a certain extent, anyway.