The main difference... .40 S&W offers a little more bullet weight and size. All things equal, the 9mm will offer more rounds and a higher velocity with standard weight bullets. Either round is extremely effective for HD/SD use. In the same frames... for instance the Glock 17/22 or the equivalent S&W M&P frames, most people will consider that the .40 S&W has a snappier recoil. From what I've read, the 9mm offers better penetration than the .40 S&W and either will do better than the .45 ACP. Then again, someone will drop a slightly heavier recoil spring into the same pistol and the .40 S&W will be about like the 9mm... just with a larger heavier bullet. For a very well trained shooter, there will be very little if any difference in the rate of fire that can be produced with either round. Shooters who do not get to fire hundreds of rounds each week will likely find that the 9mm allows them to sustain a higher accurate rate of fire.

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