Lot of good info being shared here.
I carried a 9mm for years (S&W 908)
I now carry a Ruger SR40c
Being a regular shooter and having the opportunity to shoot a few old cars in my life the 9mm was just a bit lacking.
You ask what does shooting cars have to do with self defense?
Shooting old cars gives you a lot of different materials to blow holes in.
What I noticed was that after a 9mm bullet passed through the seat it lost a lot of energy.
Now a car seat is not that formidable; cloth, foam and some springs.
A 9mm round will pass through the front and back seat and leave a dent in the deck lid but a 40 cal will exit the deck lid.
Now if I have to shoot someone in a self defense situation I only want to shoot them once.
I would carry a 45 acp but they are just to dam heavy and bulky so the 40 cal is a good compromises