OK, I have a 7-shot Mossberg 500 12ga. I would like to turn it into a hunting shotgun that can use a longer than 20" choked barrel.

From what I understand, I will have to change the barrel AND the magazine tube, correct?

Will I also need to get a new mag spring, end-knob (hopefully one with a built-in swivel/sling), and mag follower?

Any suggestions on what to get parts-wise, what length barrel and what kind of tube? This will be mostly or small-game and bird hunting, though I suppose buckshot deer hunting or with slugs (rifled barrel eventually?) could be in order in the future.