The First Amendment stops at the boundary of company property. Why should the Second Amendment be any different?
I think that you're incorrect and that there are probably many court cases that would suggest that employees must be permitted accomodation for their religious practices.
Additionall I think that if any employer tried to ban me from having my Bible, Koran, Book of Morman, etc in my car that they'd be in trouble.
So why should the Second Amendment be any different?