The auto guns adjust. The so called "fast shooters" seem to shoot in 4 round burst.
If you know how the guns are laid out on the field and you know which guns shoot fast vs. slow, you can easily de-ack a field with no damage. Same applies to ports, V Bases, and towns.
I can effectively (and usually without damage) deack a V base in 2 passes. 4 guns in run #1 and final 2 guns on run #2. It truly is about knowing where they are and how they behave.
Luck is a part of it also.
It does take training. You have to practice and practice, and practice to get it down to a science.
I rarely survive passes over medium and large fields. I can get through small airfields about 50% of the time.
I am about 90% effective on towns, ports, and v-bases in getting through deacking without getting damaged. But I haven't really practiced deacking airfields. No real reason for me to do so.
Hope this helps...