Penalty for owning an unregistered Baretta M9 at a work place if somehow found by police.?

Main setting a small shop is open 24/7, gun is kept in safe place and out of site and only kept for emergencies if ever robbed at gunpoint. Owner of store is aware of this and is ok with it. Everyone who works there has professional experience with firing a gun. Basically the question is, if someone comes in with a gun, employee pulls and fires, Cops arrive. what happens to the employee or others for the gun being there? It is not illegal to own an unregistered firearm, however what if it is found in the workplace?