As everyone else has said "doped in" doesn't seem to make a lot of sense here.
Because of the context - it being about snipers - I wonder if he said "scoped in", as in the scope on his rifle was zoomed in on the person?
It's not a common phrase, but I think I've heard it, or I can at least imagine someone saying it.
- I scoped in on the target until it was right on the cross hairs.