The 45 Automatic cartridge was the US military standard handgun cartridge for over 75 years and is still popular today amongst civilian shooters.
With a nominal bullet diameter of 0.452", the frontal area of a 45ACP FMJ bullet is 0.161 in^2 (square inches), while the frontal area of the .40S&W is 0.126 in^2, which has 78.3 percent the frontal area of the .45ACP.
For comparison, a 9x19mm bullets nominal bullet diameter is 0.355", giving a frontal area of 0.099in^2, which is 61.5 percent the frontal area of the .45ACP.
For this reason, among many others, the .45ACP is popular among shooters who feel that they may only 'get one chance to make a first impression', during a defensive shooting event, and that that shot should be as effective as possible, at the expense of the greater magazine capacities of the .40S&W and the 9x19mm Luger.
