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.