Maybe its because my artillary are crewed by raw recruits and they can't hit a barn door from 3 paces.
I have completely ceased artillary production because of the dissapointing results they give in field battles, the fact i can usually wait 1 turn to assault a city (or even better send a spy or 2 into the city to open the gate for me), and finally they slow the army down on the campeign map (unless of course you plant the army on a boat, which is not always possible).

Perhaps i should get a pantheon of mars and a top level artillary builiding place (?!?) and try some 3 chevron repeating ballistas