You will have to replace the barrel
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.
Steel
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.
Did marlin make a goose gun with a thirty nine inch barrel
See the link below
No
The BSS
Any tube that is .175 in diameter.
Lands and grooves are the portions inside the barrel that make up the rifling, which is a spiraling series of raised lines (lands) which put a spin on the projectile as it travels down the barrel. This stabilizes the projectile and allows longer range and better accuracy.
I do not personally, but yes, there is a single shot rifle that can have the rifle barrel replaced with a shotgun barrel.