I have used slugs out of a smooth bore. Around here it's common as rifles aren't allowed for deer hunting. Typically an Improved Cylinder tube works the best in most guns. My shotgun will do 4" groups at 100 yd. with Brennecke slugs.
I've read that it's not a good idea to shoot without a choke tube in the gun, could damage the threading at the end of the barrel. Additionally, if you let the gun tip over, you could deform the thin, threaded portion of the muzzle.