There is a threshold of reasonableness, if I can use those terms. It's intellectually dishonest to say that a gun is exactly the same as a book or a flag, because we all know that it isn't.
For example, some companies, such as machine shops, forbid the wearing of jewelry because it poses a safety hazard. They can then forbid the wearing of a necklace. If that necklace happens to have a crucifix on it then someone can argue that the company is violating their 1st Amendment rights of freedom of religion. It's probably been tried. We have to inject a modicum of reasonableness and common sense in to the argument and not come up with extreme examples to try to prove the point.
In my example, the gun is the same as a book or a flag, only the constitutional right has changed. It seems that you have a problem with the object (gun).
For example, some companies, such as machine shops, forbid the wearing of jewelry because it poses a safety hazard.
Go back to the contents of a vehicle on company property, and don't extend the issue onto the shop floor.
There is no right protected by the constitution to keep and bear jewelry and no religion I know of requires someone to wear a necklace with a crucifix.