You will have to replace the barrel
Steel
Adding a longer barrel to a gun can improve its accuracy by providing a longer sight radius and allowing for better bullet stabilization. It can also increase muzzle velocity, resulting in a flatter trajectory and potentially more energy upon impact. However, the overall improvement will depend on various factors such as the type of gun, ammunition used, and individual shooting skills.
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.