Expand or not, the difference between a 9mm hole and a .45 is negligible. The point of initial penetration contracts to stop bleeding. That is proven human physiology. For all the talk of "stopping/knock down power", penetration is foremost.
HANDGUNS SUCK!
They provide ZERO hydrostatic shock. They incapacitate by severing nerves(CNS) or causing enough blood loss(bleeding out/loss of blood pressure) to shut down the bodies hydraulic system. No handgun round is a guarantied stopper. NONE.
A "modern defensive" 9mm round penetrates just as far a .40 or .45. (factory)Defensive rounds are designed to operate within the standards set by the FBI after the Miami incident. 12-18" of penetration in ballistic gel.
This standard accounts for having to pass through bone/clothing on its way towards vital organs/nerves.
For the same size, I can carry near twice the ammunition with 9mm as .45. That gives me twice the time in a fight before I need to "take my gun apart"* and get back to fighting.
* removing the magazine, reinserting a new one and chambering a round, means "taking my gun apart".
If you swear by the "tried and true .357", guess what.
Matix and Platt were both hit with them. They still continued to fight for a few minutes before succumbing to blood loss.
Again
HANDGUNS SUCK.