For me it depends on whether I can avoid their bullets or not. If I keep getting, hit, then I tend to sprint really fast in random movements until they shoot their sawns and switch to uzis/tecs, and when they start shooting with those, they slow down, and thats when you are going to want to shoot them with sawns.
Sometimes though, they are thinking the exact same thing you are, so it's like a matter of outsmarting them.
I suggest practicing your movements. You can be the best lead aimer/shooter out there, but if you aren't moving, they are just going to kill you first.