The debate about which caliber is best for a defensive pistol has raged on for years. The age old push and shove between caliber, velocity, bullet mass and terminal energy has no clear cut winner. Tests in artificial media such as ballistic gelatin are not convincing either. I can say with the experience of over thirty years in a major law enforcement department and having seen many police shootings that many suspects shot with a 9 MM pistol died of their wounds as did many shot with .45 ACP pistols. Fatality is not the test of a good defensive pistol and cartridge. The test is, “did the suspect stop doing what he was doing when he was shot?” After seeing many shootings I favor the .45 ACP over the 9 MM. Many of the 9 MM shootings resulted on over penetration (which is another disadvantage in an urban environment) and the suspect was not effectively stopped, (disabled) from doing what he had been doing before he was shot. On the other hand most suspects shot with the .45 ACP were quickly disabled, there was no over-penetration, and the suspect stopped his previous activities. I should say also that the ammunition used in both the 9 MM and the .45 ACP by my department were the very best premium law enforcement ammo on the market.