After moving away from 9mm ammunition following the 1986 shootout in Miami, the Federal Bureau of Investigation will be returning to the 9mm ammunition. The ammunition was blamed to not penetrate substantially in the Miami Shootout and following that incident the Bureau moved to the .10mm before settling on the .40 caliber rounds.
For close to a decade the Bureau has considered a switch back to the 9mm and more recently, ballistic testing has shown that the 147 grain Speer Gold Dot G2 “is significantly more effective than what FBI agents carried into the field in 1986.”
You can read further details here.
