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 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.
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.
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.
Leave this to a qualified gun smith.
The rifling in the barrel. These are grooves cut on the interior of the barrel that twist around and cause the bullet to spin as it passes down the barrel. The spin stabilizes the bullet and promotes accuracy.
AR 15 Uppers are used for killing various things. Uppers are the barrel of the gun. They increase accuracy and add customization towards the gun in question.
Typically the crown of a gun is on the muzzle end of a rifle. It is the recessed area at the very end of the barrel. Poor crown cutting can be the cause of many accuracy problems.