Exotic intrinsic > Artifact perks > Subclass verbs (iE volatile rounds or radiant) is the general override order (usually you cannot have 2 anti-champion properties on a single weapon). One of the exteptions that was specifically listed by the Bungies was Chill clip, that technically should always work. This season, it behaves weird with flint striker's radiant where you stop overload champs' healing but will not stun them correctly - meaning it is not always quite the case.

Unfortunately, Anti Battier properties don’t work that well with other Champion Effects. Unstoppables and Overloads are a different story with a Anti Barrier Weapon (because even with Anti Barrier Rounds, Jolt or Ignitions can stun them)

Its complicated, there are effects from the 3.0 verbs that can stack with intrinsic/mod based anti champ effects, but volitile isn't one of them. Volitile doesn't break shields itself, but having the rounds grants your weapon the anti champ effect to break shields. If your weapon already intrinsically, or by artifact mod, have an anti champ effect, it wont be overwritten for the anti barrier effect from volitile. This is a bit different from something like chill clip, which applied slow and freeze, who's stun effect isn't handled by your primary fire, but a secondary effect in the gamespace, which is why its a valuable perk, letting any weapon deal with overloads and unstops without blocking your weapon applied anti champ effect, letting you stack radiant and deal with all three types at once. Hopefully that helps give a better understanding of how the system works, but to repeat again for your question, if you have an anti overload artifact mod for a weapon then get volitile rounds, it WONT stun anti barrier champs, as it wont override the anti overload effect

No. I believe artifact mods are overridden by both ability and intrinsic champion mods. Here's the hierarchy: Intrinsic: Exotic weapons and armor that grant anti-champion capabilities. --Will always override...-- Ability: Any anti-champion mod granted by abilities such as Suppression or Radiance. --Will always override...-- Artifact: Anti-champion mods from the seasonal artifact. I think this is how it goes. Somebody correct me if I'm wrong.

Bungie: No…too much fun.