When I worked at a gun shop, people would ask what the 'best' caliber was. My answer was always "The best caliber is the largest caliber you can reliably and accurately put on target."
"Stopping power" is a myth. As Ribit said - what you hit them with is less important than where you hit them.
I was working at the local range, and a couple came in. The young lady was looking at the guns for sale, and we had a cute little Ruger SR-22 in 'tactical raspberry', and she was oooing and aaahing over it. Her boyfriend frowned and said, "You want something bigger. That's not gonna hurt anyone."
I looked him in the eye and asked, "I shoot a 22 all the time. Wanna hold my targets for me?"
He got indignant. "No way!"
I smiled sweetly and replied, "Well, you did just say it wasn't gonna hurt..."
His girlfriend doubled-over with laughter, and he grumbled and walked away. The guys working the sales floor with me managed not to bust out laughing until after the couple had left, and there were high fives all around.

What is the point of rules that are not enforced?