I think last night’s full auto mode was more for the living than for the walkers. Hit the right spot and you take them out of the fight (and also provide a snack distraction for the walkers).
When the termites were shooting at Rick's group after they tried to break out the first time during that yummy BBQ snack, I got the distinct impression they were shooting at the ground all around the group in an effort to herd them and make them surrender. Nobody could be that bad a shot unless it was deliberate. Especially from the rooftop perches.