First there was 10mm Auto. The FBI chose a watered-down loading — a 180 grain bullet at 975 fps instead of more like 1,300 fps — and a large-frame Smith & Wesson pistol through which to shoot it. By the time the contract went through, Tom Campbell, a Smith employee, had realized that the powder capacity of the 10mm’s 25.2 mm-long case simply wasn’t necessary to achieve this same velocity. And that’s how the .40 S&W cartridge with its shorter 21.6 mm long case was born.