For many years it was generally accepted that the M1911 Automatic Pistol replaced the .38 caliber revolvers used by the U.S. Army. This after the failure in stopping power of the .38s as used in the Phillippines campaign. And that's true, more or less. There was a long birthing period in the meantime, though.
Consider this: The .38 revolver was adopted in 1892 as a replacement for the .45 caliber Colt Single Action Model 1873. Though the Colt was chambered for the .45 Colt cartridge (M1873) ammunition issued after about 1875 was the .45 S & W round (M1875) though the Smith & Wesson revolvers had been sold off as surplus.
As the Army entered into the Spanish American War of 1892, the inadequacies of the .38 became apparent. The ammunition for these guns was the blackpowder round, the .38 Long Colt. The .38 Long Colt was a progressive developement of the earlier .38 Colt Navy cartridge. Hastily, the Army recalled older Colt Single Actions from reserve stocks, and purchased a number of M1902 Colt .45 revolvers. The M1902 was the rod-ejector double action revolver of 1887 vintage.
.38 Long Colt cartridges:
.45 S & W cartridges (M1875):
These .45 revolvers remained in use, along with the .38s, for a long time afterwrds. In 1900, the U.S. Army Ordance convened a board to select a new sidearm for the U.S. Military forces. A series of tests and requirements were drawn up, and firearm manufacturers were invited to submit samples for testing. The manufacturers were eager, the Army not so much so. By 1906, not too much progress had been made. So many models of handguns had been submitted, and so many different calibers, that the Army designed two cartridges, one a revolver round, the other an auto pistol round, of .45 caliber to be the minimum acceptable round. In the meantime, the Army needed sidearms.
In 1909, the Army purchased the M1909 revolver for interim use. This was the same as the commercial Colt New Service revolver. Two variations are known, the Army Model and the Marine Corps Model. The Marine Corps model has a slightly smaller grip, and as an ex-GI, I'm biting my tongue to keep from offering my opinion as to why this was. The M1909 revolver was chambered for the .45 Colt cartridge, but the M1909 ammunition was not identical to the commercial ammunition, and was never made comercially.
.45 M1909 Cartridges:
The .45 M1909 cartridge was loaded with a powder identified as RSQ, a semi-smokeless powder. At this time, smokeless powders were not well adapted to revolver use, so semi-smokeless was the temporary fix. The arsenmal produced round has a rim that is larger in diameter than earlier .45 rounds. This to give better purchase by the star extractor of the hand ejector revolver. The rim is large enough that the rounds can not be loaded in adjacent chambers of the M1873 and M1902 revolvers, whose cylinders are slightly smaller in diameter.
The M1909 revolvers continued in service as the newer M1911 auto pistols slowly came into the ranks. (As did .38 revolvers.) With the coming of World War I, a frantic search for manufacturers of the new auto pistol was made, yet the supply still could not be met. At this time, Smith & Wesson, already making a military revolver for the British, produced the M1917 .45 ACP Revolver for use by the Army as an emergency measure.
.45 ACP cartridges:
While many of the original M1909 Colts were sold as surplus, many were turned back to arsenal and converted to M1917 configuration. This was accomplished by replacing the cylinders of these guns.
Today, the M1909 and M1917 revolvers are collector's items, with even more rare are the converted M1909 bearing both M1909 and M1917 markings.