In years gone by, it was not unusual for Chief Constable Posts to be filled by retired senior Army officers. In the '50s, we had a Major Egan (?) as Chief Constable in Middlesbrough.
"Experiences of an Irish R.M.", by Somerville and Ross, was a very good read (and was later turned into a successful TV series). Of course, Resident Magistrates (in the days when Ireland was ruled from Dublin Castle) were not exactly the same thing, but the priciple was the same.
D.