IIRC, the FBI went to the 10mm.
The other problem is that the adage from "Fistful of Dollars" turned out to be correct.
�When a man with a Mini-14 meets a man with a S&W snub nose, the man with the S&W snub nose is a dead man.�
Plus, the FBI was dealing with two very tough, fatalistic individuals who had already cold bloodedly murdered several people around the South Florida area.
I was working in Miami at the time and remember the newspaper accounts of the bank robbers and killers before the final shootout.
One way they would get their weapons was to go to some out of the way place in the boonies where someone was target shooting, usually along some canal bank in pancake flat S. Florida. They would wait for the person to empty their firearm, then rob and kill him and take his guns and his vehicle.
IIRC, the big lead that broke the case was them failing to kill one of their victims. They forced him into the canal and shot him but he survived. His subsequent description was what finally allowed the FBI to identify the two killers.
