I was just about to start playing halo 3 co op with my mate when I just noticed that I got some side black bars… it’s strange because the halo pause menu is totally fine. If I try to play halo 3 solo is also cool , never got this problem on halo cell or halo 2 while playing co op
Is it split-screen? If so I’m pretty sure that’s used to maintain aspect ratios and FOV. As far as I know there isn’t a way to remove them but I may be wrong.
yeh it is 
That’s the way the screen looks while playing co-op in MCC. Nothing wrong with your settings or anything.