Strictly speaking it would lie with the armourer and Assistant Director. Firearms are treated separately from other props because they are dangerous.
I mean. "Freak accident" is a very generous term.
Allegedly, there had been three misfires and minor injuries from guns on set earlier in production, the union crew had complained about low pay, long hours and, critically, a big lack of safety on set and things got bad enough that they walked off the project.
Rather than address any of those issues, the production hired a bunch of non-union local guys and forged ahead and now the cinematographer is dead and the director is injured.
This is literally the kind of stuff that prompted the IATSE strike.