For me, the ballista feels "stiff" once you're scoped in. This can make it difficult to drag-scope with, unless you're using a very high sensitivity.
This is especially noticable if you compare it to the DSR, which has a very "fluid" feel once scoped in.
I wondered about this because I've had a few assists by dragscoping and I'm convinced I should have got a kill by seeing where my shot landed on my screen so I rarely bother to dragscope with the Ballista. It feels much easier with the DSR.
On slums I am guessing your talking about from the graveyard to the dumpster when you say across the middle. If you think that is long range then theres your problem. Thats like 50 yards max. How do you feel you should have an advantage at that range. The ballista works fine for me. Almost always one hit kills.
the entire time i was getting my camos i played hardcore with the iron sights. makes the process easier/more fun. i really really really dont like the ballista.