I'm not a security guard, nor do I play one on the internet, but in Colorado non-LEO security is restricted to carrying a wheel gun. That may have changed but that is the way it was a couple of years ago.
I have a 2nd job as a security officer (in Colorado), the armed security officers that work for the same company carry M&P's & Glocks and a few other brands that each armed guard supplies. I have not seen anyone carry wheel guns, not even the armored car folks. When I got hired they asked me if I wanted to go armed and asked what gun(s) I would carry. I told them I have S&W M&P .45 F/S and M&P 9c, the Co. said they were ok.
I am not armed due to I work for them part time and I am not assigned a permenante location (Co. rules state you must be assigned a permenante location in order to be armed.) A couple of times they have sent me to locations to protect unknown amounts of cash and to protect a person in the ICU at a hospital the Co. felt was in danger from unknown persons wanting to finish him off since they didn't get the job done the first time. When I inquired what I am to do should someone want to complete the job and finish the guy off, I was told to radio the armed security officers a floor below and on the other side of the hospital and they would respond
to my cries of HELP! As for the $$ situation, I was told I am there to observe and call 911 should something go down (this $$ institution had been held up 3 times prior), I no longer do those assignements because I will be fired for helping the robbers take the $$ and allow the supposed killer access to the person.