When I'm shooting I notice that when I first aim I will typically lock onto a target. Locking onto consecutive targets after that (keeping aim pressed) is intermittent, sometimes it will hop to the next person, other times it will drift into thin air and I will have to manually aim.
I looked in the options and noticed three settings: Auto, Manual and Traditional (I think). I'm using the default (Traditional) but having only ever played previous GTA's on the PC (always manual) I don't really know what Traditional means. Auto-aim appears obvious, and I'd like to avoid using that if I can, but I'd like to know the techniques for using Traditional.