OK. I wouldn't don't it. OSHA is intent on forcing everyone to transform into virtual candy asses anyway. Folks are required to get training for everything now, from using a pencil, to packing cardboard boxes, so it's understandable.
If there's a shooting, and there was no anti-gun policy in place, then the business is in violation of OSHA safety regulations. Being in violation would undercut their defense against any lawsuit, so of course any plaintiff's lawyer would ensure that a complaint was filed with OSHA. The insurance companies, to protect their position in such a scenario, would mandate a "no guns" policy as a condition of getting insurance