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Do the piercing ammo power and the piercing mod for weapons stack or does one take precedence over the other?
They do not, reading over the mod's specifications and uses makes for no damage boost whatsoever. This may be patched soon.
I have tried this and confirmed on the Xbox.
They don't interact
The piercing mod does 60%(default) less damage
But the piercing ammo does not suffer a penalty
Required, but never shown