DOJ, FBI Select Federal Ammunition 5, 56mm Service% 26 Training Ammunition. Designed specifically for law enforcement needs, the DOJ-FBI recently selected the Federal Ammunition Training Service %26 in 5, 56 of NATO. The FBI has selected Glock Gen 5 pistols in 9mm as a service weapon. There has been much speculation about the reasons for its change in calibre.
There's a short answer, science. They did the research and they did the calculations. We know that the FBI SWAT uses the SIG Sauer P226 in 9 mm and the custom Springfield M1911A1 in inches.