I haven't found anything conclusive from my searches online(cant even seem to find anything on it, just deadeye in general)
Does it affect each individual pellet? Does it affect the whole thing? I'm really dying to know.
I don't have concrete evidence, but I would like to think that if Deadeye activates on the pull of a shotgun trigger, that each individual pellet is affected. I love Hunted and the default Hunted classes get Deadeye on their Specialist streak. As I was playing Hunted I was using every gun in the world. One really memorable moment on a Hunted match on Octane I had worked up to Deadeye and was using a shotgun, either the MTS-255 or the FP6 I believe. I was running around the toppled truck container in the middle of the map blasting people and I did see the activation symbol for Deadeye pop up on some less-than-direct hits. I feel that, without Deadeye activation, I would not have hit enough pellets for the kills that I got. Again, no concrete evidence, but it is my limited experience that the activation is on trigger and the damage boost is given to each pellet.