There seems to be a bug with the mod unstoppable hand cannon in the current nightfall ordeal.
This has happened to me twice now. The mod just doesn't work. I'm waiting while aimed down sights until the text comes up and I hear the noise, but it does nothing to the unstoppable ogre champion. It has happened in the past with a Dire Promise and I seemed to fix it by changing the mod then putting it back on. It just happened again today with Kindled Orchid, but this time I had just put the mod on the gun before joining the nightfall. I just exited the game instead of testing if I could put it on in the game and have it work.
Please, if you are going to force us to use these un-enjoyable game mechanics, can you fix this? It's pretty annoying.