Your briefcase is your personal property, and an extension of your personal property as is your car, would you then consider taking a loaded gun into your workplace in your locked briefcase?
You have no right to bring the weapon into the property owner's space regardless of whether you bring it in concealed (in your car, your briefcase, or your trousers) or openly. The property owner's rights allows him to set the rules for the access, use, and stay by anyone on his property, and the rule is that no guns are allowed in the space that makes up his property. The gun is in your car, and your car is on his property, which means that the gun is on his property, and that means that you have violated his rules for access and use.
You can be fired for having a loaded weapon in your car if you park your car on his property.
Exactly. Based upon your statement it appears you recognize that violating the rule may result in the loss of your job, but you are willing to accept that given the risk you'd incur by not carrying. That's the right attitude.
What we see from these employees is that they'd rather run to government than accept the consequences of their actions. That isn't conservative behavior.