In a smooth bore shotgun, you need to be sure to use slugs with fins, commonly known as "Brenneke" slugs (after the developer of the original design). The "fins" on these slugs impart a spin on the slug as it passes through the air, thereby making it more stable. The same fins allow the slug to be more easily crushed down to the necessary diameter should the choke be tighter than the outside diameter of the slug.
The other primary class of slugs are called "sabot" slugs. These are solid lead, without "fins". Sabot slugs are intended for use in rifled shotgun barrels. Sabot slugs are made to spin by the rifling in the barrel. Sabot slugs will kick like a mule through a smooth bore barrel and can cause damage to the barrel. Shotguns with rifled barrels are not choked and typically are equipped with scopes.