The FBI has given up on 40cal and moved to 9mm and the new 9mm pass all FBI penetration tests. However for most of us we won't be shooting through windshields or drywall. What FBI based on was forensic examination of wound cavities and found no difference between 9mm and 40 cal. Having killed a variety of critters who don't know what they are supposed to do when shot, bigger is always better because shot placement is difficult. So having carried since Fl first issued and having used my ccw in several occasions, didn't shoot anybody, heres what I've learned. Be aware don't let any strangers approach with out making an aggressive response upon first noticing. Never let anybody within arms reach. Now I've gone to a pocket 9mm with laser from a double stack shorty 45acp because of weight. You are not allowed to threaten people by pointing a gun, or actually intimidate someone by brandishing. So when anyone approaches in a very loud voice announce you feel threatened and for them to stay back, regardless of stand your ground retreat and place objects between you, shopping carts, poles, cars and keep repeating your warning. At the first hint of a problem my hand is on my gun in my pocket, if repeated warnings fail, out comes the gun laser on, problem usually stops before even necessary to point. However if they are high enough that won't have any effect, then it comes down to the question you should decide before you started to carry will I kill somebody. Now in my younger days I pistol whipped one unarmed purse snatcher, Double stack 45 to the head was all that was needed, I've shot twice at two thugs who robbed me at a gas station and were returning not satisfied with first take of small bills and credit cards they were both armed about 15/20 feet away I was behind a car door sitting on the rocker panel gun through open window. There guns were pointing down trying to conceal the robbery in progress at night. As they approached I put one round in the dirt right in front of each there feet, they did the whilly coyote fast retreat. I policed my brass and informed the police I was shot at since there was no witnesses except station attendant who came out after the shots. Also was followed one night into a dark parking lot by two large dark fellows I spun around thrust my hand into my gun purse and asked if they had any reason for being here they said no and left. Had three or four other kinds of gun related interactions. Criminals understand fear and aggression and I guess you could say I've always convinced them my aggression will come first. However I'm very confident of my gun handling skills, competitive shooter and I still compete weekly. Your mileage may vary. So don't ever involve yourself in someone else's violence it will end badly, and don't carry a gun unless you know you will be able to kill them, carry mace, to be really safe carry bear mace it will shoot twenty feet. Today maybe a cell phone recording the attack would help deter.