It depends on state law. I realize this sounds odd, but in some states, a sign on a building carries no legal weight, and in some states it does. HOWEVER, if you carry a concealed weapon into a building and for some reason the owner or person who controls the property finds out and asks you to leave, if you refuse to leave, you are then trespassing.
Additional: If by "public building" you actually mean a government building - NO - you may not go armed into any government building!\
That depends on the state, and also whether it is federal property. In Tennessee for instance, there is no law against carrying in a government building as long as the building isn't posted with a no weapons sign. The exception is federally owned property, but as far as state, county, or city government buildings, it's perfectly legal if the building isn't posted.
depends on your state laws
Concealed weapons are typically used for self defense.
Being a Florida concealed weapons instructor I can say for sure that Florida does.
It depends on where you are located.
Nuclear weapons weapons of mass destruction, biological weapons.
A concealed weapons permit is only a license that allows you to carry a firearm in public without it being clearly visible. Owning a firearm is a completely different set of permissions, and is usually required before you can even get a concealed weapons permit.
No.
Yes
21.
Certainly.
No
No