An 18" barrel is loud. When practicing with my Combat shotgun, I wear good ear muffs over the best ear plugs I can find to protect the hearing I have left. A 22-24" barrel is good for what you want.
As Lee pointed out, the rifled barrel is useful with Sabots. With buckshot, the rifling gets the stuff spinning and blows it all over the place when it leaves the muzzle. Get a smoothbore and check out Federal's new TruBall if you want slug accuracy.