The FBI never issued full power 10mm to any of its agents. Never. The claim that "FBI agents couldn't handle 10mm" is false.
The FBI developed its reduced velocity 10mm load about 2 years before it began issuing the pistols to shoot it.
During the FBI's evaluation of 9mm and .45 ACP ammunition terminal performance in the late 1980s, .45 ACP beat 9mm.
The FBI agent in charge of the Firearms Training Unit, John Hall, realized that a 10mm 180gr bullet has the same sectional density as a .45 ACP 230gr bullet, and wondered what would happen if they loaded 10mm 180gr about 100 fps faster than .45 ACP 230gr. They handloaded a reduced velocity 180gr 10mm load and in terminal performance tests, using Hall's personally owned Colt Delta Elite pistol, it beat .45 ACP. This is how the FBI ended up choosing the reduced velocity 180gr 10mm load.
After the FBI chose its reduced velocity 10mm load, it began shopping for a pistol. It chose the S&W 1076, which is based off the S&W large-frame 4506, but had the slide mounted hammer drop safety replaced with a SIG-like frame mounted decocking lever.
Smith & Wesson realized that many police agencies would want to follow the FBI's lead in choosing 10mm, and, for business reasons, S&W wanted to make it as easy as possible for agencies to adopt a new pistol that would shoot a cartridge that possessed the same ballistic performance but in a medium-sized 9mm pistol frame. This would allow law enforcement agencies that already issued S&W third-generation 9mm automatic pistols to easily adopt a better performing cartridge than 9mm and .45 ACP but not have to also buy all new holsters and magazine carriers, and no extra training was needed to learn how to use the new pistols. S&W teamed with Winchester to develop the .40 Smith & Wesson cartridge to be fired from the new medium-frame S&W 4006 pistol.