Bungie said this won't ever happen. The different resolution and pixel size changes the hitbox. Say if you (next gen) and Frank (last gen) were sniping at each other. Both of you pull zoom in other the other. Frank's head may appear to be 4 pixels to you but to frank you may only have 2 pixels. That's a bad explanation and the numbers are just examples but I'm pretty sure that's why they said this will never happen. I'm sure if Sony and ms didn't hate each other enough they could get cross platform but let's face it that'll never happen