The bigger caliber you go, the more stopping power you will have. From everything I have read, the one shot stop percentages are not that much different between the .45acp and the 9mm or anything in between. I say you should carry whatever you are comfortable with. If you can consistantly shoot a 9mm better than you can shoot a .40, you should carry a 9mm. I would rather hit someone 10 out of 10 times with a 9mm than 2 of 10 from a .40s&wAll good suggestions here - thanks. Now I've got someone (offline) trying to talk me out of 9mm and up to at least .40. Can I compensate with better ammo (higher grain)? Have a bunch of 115gr Winchester hollow-pt. Guy's telling me to to at least get near 140gr. True? Again, we're talking HD here, as well as CCW.