I spent over ten years working for an armored car service in the northeast. When I first started the company allowed us to carry whatever we qualified with, my preference was a 45 govt model, which I later changed to a S&W 645 which I preferred over the 4506. We were one of the fortunate few that were allowed choice. Most larger companies are so concerned with liability that they allow you to use only company issued weapons checked out at the start of shift. Usually a fixed sight revolver in 30 or 357, but loaded w/38 rnl. They don't want any perception that they have a bunch of cowboys out there.
Let me tell you though, from experiences I've seen and been involved with, in the case of a shoot your probably going to be on your own. The only reason these companies dictate what kind of weapon you carry is to protect themselves in a lawsuit, your interests are very low on their list of priorities. Most guard services don't have union representation, although when I left there was an attempt to start up an "armored guard association", but I don't know if it ever happened.
If you are looking for an "in" to get hired by a police department working armored service is a very good way to do it, well over 60 percent of the guys I worked with had used this job as a stepping stone to get on with a department. If you want to do it as a career think twice, there really isn't any overtime, and you sure as hell won't make enough to raise a family. Plan on having to keep a second job. not very good incentive to risk your life every day to protect someone else's money!