.40 S&W is a fine cartridge. With good ammo and good shot placement, you have a good channel changing combo. I would suggest go and shoot several models in that caliber to see if you're comfortable with it. Also, make sure that it is something that you can practice regularly with, because shot placement is the most important thing in a gunfight, right after having a gun
. Ammo is readily availible at Walmarts, so you're good there. I would suggest checking out
http://www.stoppingpower.net , there is a wealth of information about carry weapons and calibers there, as well as good decent folk who dont put up with trash talk on the forum. Just the most important thing, be able to shoot it accuratly.