A moral one, but not a legal one
Does the employer have a duty to protect an employee from a 'nut job' ?
If they affirmatively strip me of my ability to protect myself, then yes they do have a responsibility to do it for me.
I have to play devils advocate here. No company has a legal responsibility to protect your person from a "nutjob". The best you will be able to do is find that under US law, they have a responsibility to provide you with a safe work environment. They are required to provide you with information, training, and protective equipment for
workplace hazards.
Armed nutjobs are not a
workplace hazard, as defined by law and regulations. They may be a workplace hazard in reality, but until they become one in law (and covered by OSHA), your employer is off the legal hook.
While preventing you from having a loaded gun or any gun or even any ammo in/on company property may seem to you to strip you of youir ability to protect yourself, in the eyes of the courts it does not. As long as you have any means of self protection at all (and yes, that includes fists and feet), your right has not been stripped. It has been restricted.
And, on top of that, you chose to comply. Simply by working for the company, you are giving your agreement to their policies. And that is what will kill any lawsuit for damages, if you get shot. You knowingly accepted the risks, by working there.
SO, you know the risks, you know the rules, and you choose to work there anyway. That makes it your responsibility, not the employer.
I agree, that if they prevent you from being armed, they have the moral obligation to protect you. But not a legal one. It will be a tough case to prove, and I doubt any really smart lawyer would take such a case, hoping to win. Such a case would be a bluff, hoping for an out of court settlement, if they take it at all.
And I do work in a place where no personal firearms (or ammo) are allowed. But since we are 35 miles from the nearest town, and no one without a badge is going to get within 20 miles of where I work, and we have a full time professional armed security force, I feel fairly safe, at work.