My dads been a cop for 27 years and has carried 9mm 40 45acp and 38spcl in that time and he says that the 40 is the best compromise for knock down and round count. But something else he said it's not the size of bullet you use it's where you hit em. So whatever you feel the most comfortable useing cause it didn't matter if you have a 454 if you don't hit em right it won't stop them