Margin of error is the little extra to reach the vitals if you have to punch thru more than a broad side shot or needs to break a shoulder and still exit or if you are slightly off mark and only hit the back of 1 lung. High shock small diamiter and penatration cal some times lack it .
I agree that no bullet will bust brush and still get to the target ,not even a 12 gau slug will do it .
IMO a brush gun is a shorter barreled quick handeling rifle not a brush buster