No it’s not, stop playing dumb. A gun seller’s job is to sell guns. Even if a potential school shooter buys one, the gun seller has done their job. Who failed perhaps is the person who issued a gun license to a potential school shooter, but then you can make a case of whether potential school shooters can be detected before the act.

An armorer’s job is to make sure guns on set are handled safely. A revolver which was under her supervision has not been handled safely, and as far as we know it was her and precisely her mistake that left the live bullet inside the barrel, therefore she failed at her job and is culpable for the death. This in no way compares to a gun seller.