It may or may not depending on the ammunition used.
Depends on how bad it is. Have it examined by a qualified gunsmith.
No
It may or may not depending on the projectile and velocity.
The barrel has deep grooves milled into the outside of the barrel. Called flutes, the grooves reduce the weight of the barrel, yet leave it a stiff barrel with very good accuracy.
Rifling a gun barrel causes the bullet to spin when it comes out. This makes the bullet fly more straight and greatly improves accuracy. Much like a spinning football spirals.
A rifle is a long gun with a rifled barrel designed for accuracy at long distances, while a shotgun is a long gun with a smoothbore barrel designed for shooting multiple projectiles at close range.
A BB gun uses compressed air to push a round steel "BB" out of a smoothbore barrel. It does not have the power, range, or accuracy of a firearm.
Leave this to a qualified gun smith.
You did not specify the make of your Model 300 (there are several), but in general, a 12 g shotgun CAN fire slugs from any standard barrel. You will get better accuracy from a slug barrel, but you could use any barrel OTHER than an "Extra Full", or a shotgun with a variable choke.
Bullets spin when fired from a gun due to the rifling inside the barrel. Rifling consists of grooves that twist around the inside of the barrel, causing the bullet to spin as it travels down the barrel. This spinning motion stabilizes the bullet's flight and improves accuracy.
Bullets spin when fired from a gun because of rifling, which are spiral grooves inside the gun barrel. The spinning motion stabilizes the bullet's flight, improving accuracy and range.
The Paris Gun was not a great success: the payload was minuscule, the barrel required frequent replacement and its accuracy was only good enough for city-sized targets.