Simplifying this to Alec Baldwin solely being an actor on the film, i'm struggling to see how he can realistically be held responsible. The whole point of hiring a specialist to set the guns up is to ensure everything is safe etc.
As producer/director/whatever, if this is in his remit, I would expect they could be held liable if they'd hired someone without appropriate qualifications, or they hadn't put procedures in place to ensure that appropriate checks were carried out. But if they had done so, again, I can't see what more they can do.
Assuming the above is all correct, the blame lies with the armourer in my view.
If the armourer wasn't suitable/trained/qualified, then is Alec Baldwin responsible? Or is it someone else's role?