You can shoot slugs or rifled slugs.

You should not shoot sabots. The concept behind a sabot is that it makes contact with the rifling in a rifled barrel and imparts spin on the projectile inside to stabilize it in flight. Since you do not have any rifling this goal will not be accomplished and the bullet will begin to tumble and fly erratically shortly after leaving the barrel.

Both standard slugs and rifled slugs are designed to use natural aerodynamics to stabilize themselves. In the case of rifled slugs the design actually causes it spin as a result of its aerodynamics,(or more appropriately lack there of).

I would field test your gun to see what kind of accuracy you can achieve. I would espect you should be capable of consistantly hitting the vitals of a deer sized target out to 50-60 yds with this setup.