Unfortunately, both of these scenarios happened as you each explained them. Brandon Lee was killed when a bullet that was lodged within the barrell ended up being fired out after a dummy round was shot in the weapon.
Jon-Erik Hexum was playing around doing a pretend game of Russian Roulette in between filming after the scenes weren't filmed up to the director's standard. The plastic/paper debris from the dummy round was ejected from the weapon and damaged Hexum's brain.
I recently began buying replica western weapons to learn how to twirl (like Tombstone or Revolver Ocelot), and it is beyond me how Hollywood film crews could not have a licensed firearm expert there to inspect and clear every weapon before every scene. After these earlier two situations with Lee and Hexum I'm not sure why they even use fully functional firearms anyway, given the accuracy of non-firing replicas which can use caps and the capabilities of modern digital editing. It couldn't cost them more than a few hundred bucks to get a very high-quality non-firing replica.
I think the big difference is that Hexum accidentally killed himself goofing off and Lee was killed actually filming a scene (believe it was the pawn shop scene).
Both tragic, but Lee was killed by negligence from the people who were suppose to be supervising the guns.
I think they bought the gun that killed Lee at a flea market or antique shop and never knew about an old bullet lodged inside it.
Been awhile since I saw the documentary, but it was something along those lines.
It’s messed up because not only did a young Lee die so needlessly, but Massee, who played Fun Boy and fired the gun, was haunted by what he did up until his own death.