Well I have worked in a level one trauma hospital for almost 20 years. With the bird shot, it is like any defensive weapon, it can be devistating or not. I have seen a case where bird shot was used at a distance with only a few pellets hitting the side of the head, but one happened to penetrated the suture of the skull and transversed the brain killing the individual. In another case, the patient tried to commit suicide with a 410 under the chin and lived. Both of those were the exception.
Birdshot can be very effective and I do not hesitate to use it. That being said, as with any defensive weapon, don't stop until the BG is no longer a threat.