Actually, there is an easy way to solve that.
If anything is between you and the squares you are targetting for autofire, those squares receive a cover bonus. You're trying to hit DC 10. IF you happen to miss due to the cover (be it a person or object) then that object has to roll a reflex save instead of the original target. To represent some bullets still getting through, you can say that whatever is in the original targetted area still takes damage, but only half.
Basically under the cirucmstances they get evasion for free, or something like that.
As for Bullets going further..hows about this.
Elongate the area by two additional 10x10 areas
In the end you'll have a 30x10 area lengthwise.
The target is the center 10x10 area. Anything beyond in in the tertiary 10x10 area, takes 1 die less damage.
Anything in front, grants cover as i mentioned above.
that might work for a solution.