...was the forerunner of the renowned .38 Special and was the reason the .45 ACP was invented.
Developed in 1872, the .38 Long Colt, or .38LC, was originally a BP round. When smokeless powder hit the scene in the 1890's, many rounds were converted and the .38LC was included. There was also a .38 Colt, which was just a shorter version and a rimfire .38 which pre-dated that. The .32 caliber offerings come to mind as well.
When we defeated Spain in 1898, and gbought up their colonies (Cuba, Puerto Rico, Phillipines), we purchased an insurrection as part of the deal. The Phillipinos had been rebelling against the Spanish for some time and were not about to become the puppets to another Imperialistic power.
When we went to the Phillipines to "pacify" the good people of that region, fightin broke out in 1899. our service men carried along a nice revolver by Colt, the New Service, chambered in .38 Long Colt, which had been adopted in 1892. A very nice, old-world firearm, it had one problem: It was ineffective, as such things go!
With a pitful (by modern standards) 195 ft/lbs of muzzle energy, it had little effect on crazed jungle tribesmen who went into battle hyped up on drugs! The round nose lead bullet, at a pretty meager 700-800 fps was about like throwing rocks, or so it is reported. An immediate cry went up for something better.
For brief time the Army switched back to the old .45 LC. So it comes as no surprise that the Army, after much trial and error, determined that a bigger bullet at around 850 fps was the real answer and the .45 ACP was born. Put up in an auto loader (our first) it gave you 1-2 more shots of a real thumper, a manstopping caliber par excellence and it remained the US handgun service cartridge for about 80 years.