I am normally very skeptical of tort reform, but protecting the marginally profitable firearms industry from nuisance suits is REASONABLE. Unless a gun is defective or unreasonable dangerous in some way, they should not be liable for the criminal misuse of one of their products. This is something that is generally accepted in torts. I suppose if a manufacturer made a gun and marketed and sold it to mental patients, criminals, and low lifes, they may have a case.
I am genuinely sorry for the family members of the victims, but the only party responsible is dead.