Depends on where you are, what land, and what you are shooting.
In the U.S. it is not illegal in any state that I know of, as long as you are in a location where it is otherwise legal to fire the gun.
Most pistols will fire regardless of whether you're in the AZ desert or not.
potato guns are not allowed to fire projectiles. that would make them illegal, but you can have one
No. You cannot fire anything other than a .380 ACP cartridge out of a .380 handgun.
No! DO NOT ATTEMPT!
No, it cannot.
Unless you tell us what type of handgun, the question can't be answered. Single Shot? Revovler? Semi-automatic? Black powder? Center fire? Rim Fire?
In most circumstances, no. The bullet is too large to chamber in the handgun.
Depends on where you are, who the land belongs to, whether you have permission, are you in a safe area etc. We cannot answer without more information.
no it is not illegal to have a fire on a Sunday
Yes
No. You can only safely fire the correct caliber from each handgun. For instance, for a .40 caliber handgun, you can only fire .40 ammunition. There are some minor exceptions, mostly in revolvers. For instance, you can fire .38 special or .357 magnum from a revolver that will fire .357 magnum, but not the other way around.