Why smooth bore gun?

Answer 1

some firearms, like shotguns, have smooth bores. Due to the wide variety of different projectiles a shotgun is expected to shoot (from a fine dust-like shot, to a single, solid chuck of lead called a slug), the best type of bore, in this case would be a smooth bore.

With a rifled barrel, you want to shoot ONLY a projectile of a specific diameter. This way, the rifling grooves can grab the projectile and impart a spin to it. With a shotgun, the presence of rifling would only confound things, and likely the rifling would get worn out / damaged by the wide variety of projectiles sent through it.