HP slugs have the same damage profile as Rifled Slug, though adding 2 Extra Damage Rolls (3d6 -> 5d6).
Much like hollow-point bullets, shotgun slugs which feature an open front cavity will massively expand upon impact. Against even lightly armoured targets, this will result in a harmless disintegration of the projectile. However, if such a slug hits a soft target, it will transfer its energy in a wide cone throughout the body, shredding the interior rather than leaving an exit wound.