To hit them you should fire when you are generally in the green or atleast past 1/2 way on the aim bar.
At marksmen 20 with Q10 sling is slow to fill the bar, at marksmen 40 with q20 sling is quicker obviously but still takes dozen seconds to get it over 50% aimed.
With arrows the aim is twice as slow, as i would personally stick with a sling and rocks until you get a decent bow and high marksmenship.