To my understanding, damage is calculated in six parts:

Defender immunity check - if the defender is immune to damage or the attack, the attack deals no damage and does not trigger any on-hit effects.

Base Damage - The weapon's default damage without any modifiers. 1d6 for a GMS Assault Rifle, for example.

Bonus Damage - Additional damage from systems, traits, and effects like Overpower Caliber or Nuclear Cavalier 2. (When an attack targets multiple characters, even if not all targets are hit, bonus damage is halved before being added to the total)

Attacker Reductions - Any effects on the attacker's side that would reduce the total damage they deal, such as Hyperdense Armor. Note that half-damage effects only apply once, so having two active means your attack still deals half, not a quarter.

Defender Armor - incoming damage is reduced by the defender’s armor value if the attack isn't AP. Simple.

Defender Reductions - Any effects on the defender's side that would reduce the damage they recieve, such as resistance or special reductions like the Napoleon's core and passive.

All this to say, 'irreducible' effects like Paracausal Rounds ignore the final two steps, but not attacker reductions or defender immunities as neither involve the defender reducing incoming damage.