I was part of a conversation yesterday with a group of LEO's concerning the best self defense ammo. The conversation ranged from 380, S&W 40 cal, 357, and 45 cal. I hear the term "knock down power" or "stopping power." Knock down power is not really a correct term as the energy needed to knock someone down (as seen in the movies with flying body's after being shot) would also be transferred to the shooter. I spoke about the fact of the matter remains, the larger area that the bullet can damage, the better chances you have to stop or terminate the aggressors abilty to continue the attack. Of couse bullet placement has the most effect on the out come also, so marksmanship skills to count.