What did, or does, need have to do with it? I think the sales numbers would speak for themselves regarding the 40's validity and utility. I've found that proper handloads put the 40 into respectable performance. It is a fine cartridge, and the only thing I can see that people have to bitch about is that it isn't a bit faster, or maybe a bit fatter. I'd love to get 10mm numbers, but top end 40 loads, such as a 180 cast flat nose at 1200, are the limit of the snap I want from the platform. And really, no more is needed.
The .40 S&W doesn't not have to fly at 10MM speeds for it to be effective for self-defense. A 180 grain bullet at about 1000 FPS and you're good to go.
