When fired from a sling, the attack slide gains +2 To Hit and +2 damage as Blunt and can crit on a natural 19-20. Its use must be declared (turned in to the DM) prior to making the attack slide.
This stone is not subject to further effect from Oil of Smiting. However, if some other crit-enhancing magic can expand one’s crit range beyond 19-20, use whichever crit range is wider.