I'm looking at purchasing the S&W Shield in the near future. Can't decide between 9mm or .40 cal. I have an old (circa 1987) S&W 669 in 9mm, but it's a double stacker. Want the more compact Shield. I have never owned a .40 cal, but per the ballistics info it's only got about 50 more ft-lbs of muzzle energy over the 9mm. The average .40 cal around 400 ft-lbs, 9mm around 350 ft-lbs. The 9mm is higher velocity and a bit more flat trajectory. I know this debate goes on endlessly, but one thing I might also not like about the .40 cal is the sharp impulse kick vs the more docile 9mm. People say it's easier to keep point of aim with a 9mm during multiple shots. Plus I already have lots of 9mm ammo; don't know if I want to start investing and stocking up on a new caliber. But if the .40 is worth getting over the 9mm I would go that route. The Shield looks like a sweet pistola ... checked one out at the local gun store and it fit like a glove to my hand.