I forgot which website I read this on but it was from a link on this forum. The wound channel depends what type of tissue the bullet strikes. Most tissue is elastic and will spring back and the permenant cavity will only be the diameter of the bullet, if not smaller. The site mentioned that some types of body tissue are not elastic and tend to tear rather than spring back. The liver is one such type. I think it said that the brain was another. For other types of tissue, I think the permenant cavity will be the bullet diameter or less. If it is a very high velocity round, the stretch cavity may be bigger than the bullet diameter since the tissue may expand more than its ability to spring back.
The ATF should be a convenience store instead of a government agency!