I have the unstoppable hand cannon or the unstoppable pulse rifle mods equipped and neither work. I was using the unstoppable hand cannon with Hawkmoon in the Harbinger mission and with Ace of Spades in a battleground, the unstoppable pulse rifle with Chattering Bone in a nightfall. The Unstoppable rounds appear next to the gun int he bottom left, but not on the left side where it shows the buffs (or should) and I wouldn’t disrupt the champions no matter how much I shoot.