Steel
You will have to replace the barrel
Not a generally used term- it would be the portion of a gun that holds the barrel. Not present on all guns.
Any autococker threaded barrel.
The tool used to bore a gun barrel is called a "bore reamer." This specialized cutting tool is designed to precisely remove material from the inside of the barrel, creating the desired dimensions and finish. In addition to bore reamers, other tools such as rifling machines may also be employed to create the grooves inside the barrel for improved accuracy. Proper machining is crucial to ensure the barrel performs effectively and safely.
Oxidation of the steel of a gun barrel.
Did marlin make a goose gun with a thirty nine inch barrel
No
The BSS
See the link below
The first gun with a rifled barrel (a rifle is any gun with grooves cut into the barrel to give the projectile spin) used by American forces was the Kentucky rifle, used in the revolutionary war.
I do not personally, but yes, there is a single shot rifle that can have the rifle barrel replaced with a shotgun barrel.
Basically they are the same. But a long Barrel and a long shoulder stock make a gun a Rifle.