I believe the FBI selected the 3” M13 (and M65) partly because it has a longer ejector rod than the 2 3/4” M19/M66. They also chose a fixed sight gun because from what I’ve seen of the FBI an agent would be required to submit a memo if they wished to adjust the sights on an issue revolver. I don’t know if a separate memo would be required for a change of windage and elevation but I could foresee it bogging down the whole agency in short order.
Minor correction. The M19 available with a short barrel at the time the FBI adopted the 3" M-13 had a 2.5" barrel. The 2.75" K-frame Magnum is a more recent offering/option.
Dave