Question from someone who doesn't know a lot about guns.
Why can they make firearms with barrels a slightly smaller caliber for movies so live rounds don't fit?
The gun they used wasn't a "prop" in the sense it was a replica.
They used an actual, commercially available, working firearm and called it a day.
There was no reason whatsoever to have live ammo on set like that. No one with any good judgment should have wanted to have it around anywhere.
Example: You can easily get "snap caps" that can be made to look like bullets (good enough for the camera) but be just solid, inert metal. These are routinely used for training with working firearms.
You could even have easily manufactured real looking ammo [in the prop department] using actual bullets, and actual brass cases, but no primer and no powder so that they are also inert but also easily distinguishable, but difference hidden from the camera.
Or just epoxy bullets (meaning just the lead projectile) into the revolver cylinder and it's inert...
They didn't bother creating any non-functional replicas. It's like going to cosplay at comic con, but cheating and using an actual firearm and real ammunition. The height of stupidity.
Of course you need competent executives hiring competent staff who create competent procedures, then you train the people handling the firearms on the processes, and have all those above then ensuring procedures are followed. Baldwin failed in two ends here.