It actually came about because of a change in Sheriffs. One of our Captains, who was very gun savvy, got himself elected Sheriff. One of his first acts, in the first week of his term, was to make it policy that if you wanted to carry your own sidearm, and could qualify with it and furnish your own practice ammunition and duty ammunition, he would allow it. He set realistic parameters as far as caliber (.38/9x19 minimum-.45 cal. maximum) and brands. Subsequent Sheriffs followed that policy until about a year and a half before I retired, so I was able to carry my .41 for almost 20 years of my career. Fortunately, I was behind a desk when the last Sheriff I worked for mandated the issue firearm, which I hated. I won't disclose what it was, since that would be way off topic, and start another war. Our department had just over 600 sworn officers.
As for the type of ammunition allowed, the County Attorney's Office always got involved, which was the big stumbling block, hence the .357 Magnum ban. Attorneys watch too much TV, and make decisions accordingly............
Fred