No harm except the money you spent!
A saboted slug requires spin stabilization, which you won't get with a smooth bore, leading to loss of accuracy. You can get a choke tube with rifling for most screw-in systems, and that imparts a spin at the very end of the barrel. I've heard they work fairly well, even though we're talking about a comparatively short section to grab it and turn. What gun/barrel will you use?