Unfortunately it depends a lot on your ping. That can have a bigger impact on your shooting than bullet travel time.
You could try aiming at their feet and looking for the dust kicked up by your bullet. How far behind the target that is will tell you how far in front you need to aim next time.
If this is truly the case, and tripwire haven't implemented any ping compensation, then that's digusting...
i don't play on anything higher than 80ms
Seriously, what's the point in adding ballistics for realism if it's tottaly destroyed because of pings, it just KILLS the immersion....