Don't use full zoom unless you really need to.

If you're going to only kill engineers, I suggest you take the ramjet and stand on ground of about the same height as they do (don't be above, don't be below them), since you can hit several targets with one shot (my personal record is 4 engies, w00t ) and engies tend to be grouped.
For anything else, take the regular rifle. It has a faster firing rate.