I would say they are jointly responsible for the bad outcome.
As for the armorer, its really a 'you had one job' issue*. If you dont know how to source and recongize the correct blanks and dummies from a commercial supplier of theatrical props, you have no business signing up for that job, doesn't matter who your daddy is.
Same for the AD who was supposedly in charge of set safety. If that is your job, maybe you need to supervise an inexperienced staff member to make sure that the prop guns get loaded from a box of commercially made dummies, not a grab-bag of random reloads.
Yes, ultimately it was Baldwin who failed to double-check the the shoddy work of the three people who came before him, but in the end, the responsibility falls on all of them.
* yes, I know, she also did props, but for the job description of armorer, he job was to make sure for a cold gun not to go 'bang' and for a hot one to do so.