Digging around in the basement, I looked at some of my old books on various subjects and noticed a couple interesting factoids on this subject. A book on the Spanish-American War written in 1904 by a couple of the generals involved in that fray used the term "revolver" for the standard military issue handgun of that time- a double action revolver. Later, when discussing the situation in the Philippines, the Colt Single Action Army was referred to as a "pistol". That seems to jibe with some of the dime novels I have that were written in the same time frame.
The use of "revolver" really seemed to take off with the pulp detective novels of the 1920s and especially 30s. If not referred to as a "gat" , "heater", "rod", or other slang, then revolver was the term used for a wheel gun. Pistol was most often reserved for a semi-auto handgun in this era. As an aside, "automatic" or "auto" began to see wide usage in place of "semi-automatic" during this timeframe too.
With only the dozen different sources I looked at, I couldn't find a definitive reason for a revolver to not be a pistol. The best I can guess is "revolver" grew to define a double action revolver while a single action was often called a "pistol". It is probably the fault of the British for this conundrum- they devised the Webley-Fosbery Automatic Revolver around 1900 which greatly blurred the lines if not erased them.
