You’re correct. Manufacturing defects or design flaws can lead to lawsuits. Worn or altered parts would not be something brought against a manufacturer.
With revolvers probably the most well known example is Ruger’s single action revolvers. Even with the fix being implemented in the 80s and applying to pre-1973 models, they still post notices about it to have an old one sent in for a retro fit.
My dad bought a used bolt action rifle that he figured out had a slightly out of round chamber. He sent it back to Remington who rebarreled it free of charge, but charged him $75 to replace the trigger, as someone had altered it and they won’t send a gun altered in that way back out. They manufactures are certainly keenly aware of liability for a faulty product.
Because things done on movie sets are not transferable to real world gun handing. When you have people shooting at each other, flagging people with muzzles, you hare violating actual safety rules left and right. Because a western outlaw or mobster doesn’t care if they flag someone.
Actors can be expected to know and operate a firearm safely in general, but most of them do not have the knowledge to handle everything safely with out supervision. Some may, but that is why you have experts on hand to oversee everything. The experts on hand should never had live ammo on set, nor mixed it up with the dummy rounds.
An actor my know how to properly rig themselves into a harness, or set a squib, or prep a car for stunt - but they have other duties and those should be handled by the teams of people trained to oversee and focus on the stunt at hand.
Brandon Lee’s accident was a tragedy due to poor process by the gun handler. The person who shot him used blanks, but there was a barrel obstruction that was dislodged afterwards. Even if the actor checked to make sure they were blanks, he had no reason to believe there was an obstruction.
I would agree with you in most other examples of real world use, as the handler is the person who should be in control and know how their firearm operates. The exception is someone receiving bad instruction or lack of supervision that results in an accident. I can think of a couple examples where the shooter wasn’t at fault.
At any rate, I suppose the investigation will play out and we shall see if there is negligence charges brought or not.