Interior or exterior?
Water and aluminum foil can remove surface rust on the chrome of a gun. Steel wool can also be used to remove rust. It is advised to never use a oil to remove rust or to clean a gun.
If not properly maintained, yes.
From markings and proof marks on the gun barrels and the frame. In some cases you need to remove the barrels, and look on the underside from markings.
These guns are quite old 46 to 60 years old the barrels can be very difficult to remove they are screwed into the receiver you should be a gunsmith to remove the barrel as fitting a new barrel you will have to make sure that the head space is ok. Most guns of this era had fixed barrel even the early Remington 870 had fixed barrels you will need a barrel vise barrel spanner to start with years of shooting and rust make thes barrels a challenging task
Gatling gun
First, you cannot fix the damage already done but you can repair and stop the rust from doing more damage. Take a fine sandpaper or a wire wheel or steel wool and remove all the rust from the gun. After the rust is removed, reblue the gun with a solution made just for this. Ask any gunshop for the solution you need. If the damage is to the point to make the gun unsafe to use as in holes in the barrel or in the firing mechanisms, you may need to replace the effected parts.
Rust remover is a liquid that can be used to remove rust. A homemade rust remove is white vinegar. It can be sprayed directly onto the rust.
It is not correct.
Blue is a type of rust finish
Oxidation of the steel of a gun barrel.
The original Gatling gun had six to ten barrels which rotated and fired independently. This was one of the earliest machine guns made.
It is a form of controlled rust. In hot bluing, the steel parts are placed in a hot chemical bath that changes the outer layer of steel to black iron oxide. This helps protect the steel beneath it from further rusting. In rust bluing, the steel is exposed to moist air, and allowed to rust under controlled conditions.