There's more than just bullet diameter at play regarding blood trails.
Sectional density plays a big role in this discussion. Blood trails are bigger when there is both an entry and exit bullet hole. Higher S.D. would yield more penetration.
There is bullet construction ( rapidly expanding bullet like a Ballistic Tip, vs. Partition, or monometal), lead core hardness, and jacket thickness).
There is bullet velocity at impact. Excessive velocity causes bullets to expand fully and fast, creating increased frontal cross section /frontal.area, which decelerates a bullet faster than a bullet partially expanded- with a smaller frontal area. It is possible to have a bullet with a lower impact velocity to have more penetration, compared to the same bullet at a faster impact velocity. If the lower impact velocity bullet has limited expansion ( less frontal area), it will decelerate less, and penetrate deeper. For example, the best penetrating bullet is a FMJ - which has minimal frontal area expansion, and can whistle through a game animal at low impact velocity, and cause a "two- hole blood trail."
For "blood trail" discussion ( not "drop dead right there" shot placement to the CNS) more factors than bullet diameter need to be included.
