A modified or improved cylinder should be fine with a slug, just not a sabot slug. A sabot slug should only be used in a rifled barrel. I would test it out at the range though, as some slug and choke combos are more or less accurate. I have had pretty good accuracy with my mossberg 500 with a 28 inch barrel and a modified choke shooting Federal Hydra-Shok rifled slugs.
-Sh0e