Single shot pistols, of both the percussion and flintlock persuasions, were the type of sidearm most commonly in use during the period 1840-1860. These ranged from pocket pistols up to what we now refer to as Plains pistols. They came in box lock, side lock and back lock configurations. Their calibers ranged from puny .28 caliber pocket pistols up to massive .54 and .58 caliber weapons. For example, in Worman & Garavaglia's Volume I of Firearms of the American West, Capt. Phillipe St. George Cooke refers to using a U.S. Model 1803 flintlock single-shot pistol while running buffalo on horseback in 1844. That .54 caliber round ball would do the trick, if you were brave enough...and your horse was steady enough...to get close enough to the buffalo! And the Model 1836 flintlock and Model 1842 percussion pistols were in use as well.
Revolvers became more common after the Mexican-American War. While we all like to think about the big Colt's Dragoons and the Navy model (at least, I do!), it is wise to remember that more Colt's Pocket Revolvers were made during this time period than all the others combined. In the great book on Western holsters, PACKING LEATHER, there are a number of examples of Colt Model 1849 .31 caliber revolvers being carried in "Slim Jim" holsters.
Jake