You will have to replace the barrel
Steel
Usually, yes. A longer gun barrel allows the bullet to accelerate longer and thus increases its velocity. But it is a complicated problem. A short barrel and a large powder load has about the same velocity as a smaller load and a longer barrel. A longer barrel suffers more wear. Really big naval guns were good for a limited number of shots and each shot was numbered, being bigger for every shot.
Within limits, a longer barrel means a faster bullet. It is pushed by the expanding gasses from the fired cartridge for a longer time, and accelerates to a higher speed. A longer barrel will also mean a longer distance between the front and rear sight, permitting greater accuracy in aim.
No. the longer barrel will preform better.
Oxidation of the steel of a gun barrel.
No
See the link below
The BSS
Did marlin make a goose gun with a thirty nine inch barrel
I do not personally, but yes, there is a single shot rifle that can have the rifle barrel replaced with a shotgun barrel.
To make a burn barrel last longer, you can use a metal barrel instead of a plastic one, ensure proper ventilation, avoid overfilling it, and regularly clean out the ashes to prevent corrosion.
Basically they are the same. But a long Barrel and a long shoulder stock make a gun a Rifle.